cmake: include folder and common folder is private to llama library

[mofosyne]

Issue Ticket: #11630

[mofosyne]

Not as familiar with CMake, but looks like this change doesn't quite work.

[slaren]

The public directory still needs to be public. Some examples may depend on the llama targeting adding the src and common to the include directories, and will need to be adjusted. Some tests use headers from src because they need the lower level access to llama.cpp. That's ok, but they need to add the include directory themselves.