Introduce new header file sgsn/gtp.h

It will be used to store all stuff relatd to libgtp use and GTP
protocol, similar to what we already do for other protocols.

Change-Id: I4aae35cd0ea401856cd822cb507d668350d07a89
diff --git a/include/osmocom/sgsn/Makefile.am b/include/osmocom/sgsn/Makefile.am
index dbbc0c6..fc4f597 100644
--- a/include/osmocom/sgsn/Makefile.am
+++ b/include/osmocom/sgsn/Makefile.am
@@ -21,6 +21,7 @@
 	gprs_sndcp_xid.h \
 	gprs_subscriber.h \
 	gprs_utils.h \
+	gtp.h \
 	gtphub.h \
 	gtp_ggsn.h \
 	gtp_mme.h \