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
Osmocom has maintained this program since about 7 years now, while
the original author / copyright holder has completely disappeared.
With the introduction of Osmocom-style CTRL and VTY interfaces,
the way how the program is used and configured has substantially
changed. In order to avoid confusion in terms of configuration file
format etc, let's rename it to OsmoGGSN.
Change-Id: I2da30f7d4828e185bfac1a4e2d8414b01cbe4f9d
