Always set installprefix

[ankon]

This aligns how include/Makefile and src/Makefile treat this, and fixes make pkg to not fail due to missing include files that ended up in a different directory.

Error I see on main on Fedora 41 with make pkg:

...
RPM build errors:
    File not found: /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT/usr/include/bpftune
make: *** [Makefile:52: pkg] Error 1

Looking at the install output:

+ /usr/bin/make install DESTDIR=/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT 'INSTALL=/usr/bin/install -p'
make[1]: Entering directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1'
cd src; make install
make[2]: Entering directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1/src'
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
/usr/bin/install -p -m 0755 -d /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/sbin
/usr/bin/install -p bpftune /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/sbin/bpftune
/usr/bin/install -p -m 0755 -d /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/lib64
/usr/bin/install -p libbpftune.so* /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/lib64
/usr/bin/install -p -m 0755 -d /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/lib/systemd/system
/usr/bin/install -p -m 644 bpftune.service /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/lib/systemd/system
/usr/bin/install -p -m 0755 -d /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/lib64/bpftune
/usr/bin/install -p tcp_buffer_tuner.so route_table_tuner.so neigh_table_tuner.so sysctl_tuner.so tcp_conn_tuner.so netns_tuner.so net_buffer_tuner.so ip_frag_tuner.so /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//usr/lib64/bpftune
/usr/bin/install -p -m 0755 -d /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//etc
/usr/bin/install -p -m 0755 -d /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//etc/ld.so.conf.d
echo /usr/lib64 > /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT//etc/ld.so.conf.d/libbpftune.conf
if [ /home/andreas/rpmbuild/BUILD/bpftune-0.1-build/BUILDROOT =  / ]; then ldconfig; fi
make[2]: Leaving directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1/src'
cd include; make install
make[2]: Entering directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1/include'
install -dv -d /home/andreas/rpmbuild/BUILDROOT/bpftune-0.1-3.fc41.x86_64//usr/include/bpftune ; \
install -m 0444 bpftune/bpftune.bpf.h bpftune/bpftune.h bpftune/corr.h bpftune/libbpftune.h bpftune/rl.h bpftune/vmlinux_aarch64.h bpftune/vmlinux_x86_64.h -t /home/andreas/rpmbuild/BUILDROOT/bpftune-0.1-3.fc41.x86_64//usr/include/bpftune ; \

make[2]: Leaving directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1/include'
cd docs; make install
make[2]: Entering directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1/docs'
make[2]: Leaving directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1/docs'
make[1]: Leaving directory '/home/andreas/rpmbuild/BUILD/bpftune-0.1-build/bpftune-0.1'

[oracle-contributor-agreement]

Thank you for your pull request and welcome to our community! To contribute, please sign the Oracle Contributor Agreement (OCA).
The following contributors of this PR have not signed the OCA:

• PR author: ankon
• andreas.kohn@gmail.com (@ankon)

To sign the OCA, please create an Oracle account and sign the OCA in Oracle's Contributor Agreement Application.

When signing the OCA, please provide your GitHub username. After signing the OCA and getting an OCA approval from Oracle, this PR will be automatically updated.

If you are an Oracle employee, please make sure that you are a member of the main Oracle GitHub organization, and your membership in this organization is public.

[alan-maguire]

change looks good, if you can follow the instructions above to sign the OCA I'll be able to merge it. Thanks!

[alan-maguire]

ping on this; I'm afraid without the OCA signed I'm not able to merge. Thanks!

[ankon]

Doh, got stuck last year somewhere there, and then forgot about this. All signed now!