Missing gpu.h, net.h, mat.h? Add the need to build ncnn to install directions

[schnorea]

After the build and installation of:

• MNN
• OpenCV
• CodeBlock

You also might need to build and install
https://qengineering.eu/install-ncnn-on-raspberry-pi-4.html

[Qengineering]

In this repo, three different deep learning models are compared.
However, it is possible to use only one model (ncnn, MNN or OpenCV) in your app, which will be the case in many situations.
For this, we have provided instructions in separate Readme files, on how and what to install a single model on your RPi.
If you want to use all three you are absolutely right, you need to install both MNN and ncnn in addition to OpenCV and CodeBlock.