Move the module cmake code to its subdirectory for better organization,
and remove superfluous include_directories() that are not recommended
to be used (as it populates all other targets with those directories,
most of which don't need it anyway).
Slightly clean up the module by removing headers to unused dependencies.
The GTP module has a dependency to 4G and 5G RLC, which should not exist
(the RLC modules have a dependency on GTP, not the other way around),
but in this commit, make as little modifications as possible.
Robert Schmidt
be4461ccca