Changes

libgtp: move includes to osmocom/include/gtp

Move all includes from /usr/include/….h to
/usr/include/osmocom/gtp/….h to be more consistent with other Osmocom
projects, and to not "pollute" the top include directory if we add more
header files.

Also the new directory structure makes more obvious, which headers are
public and which ones aren't.

Adjust libgtp.pc.in so both #include <gtp.h> (legacy)
and #include <osmocom/gtp/gtp.h> can be used.

Related: OS#6373
Change-Id: If7e01c61168819bf7120667344e40c857da5490b