fix: add missing #include <cstdio> in xlings.cppm for stderr

The previous commit removed <cstdio> during cleanup but stderr is still
used in ensure_init(), ensure_patchelf(), and ensure_ninja() warning
messages.

Author: Sunrisepeak

src/xlings.cppm

@@ -8,6 +8,7 @@
 // `mcpp.pm.compat`. It must NOT import mcpp.config or any other mcpp module.
 
 module;
+#include <cstdio>      // stderr
 #include <cstdlib>
 
 export module mcpp.xlings;