T: isolate generated .h files when using make

When using the make system to generate T files, if you change T_messages.txt you may have problems compiling the gnb with ninja.

(This is a problem faced by programmers/hackers of openair, not general users.)

Let's fix that!

(putting both 4g and 5g labels since it touches the build system)

(maybe "build only" would be enough, not sure, so I put it all)