Move osmo_gsup_messages.[ch] to libosmocore

This requires the corresponding commit in libosmocore.
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 7889d5f..6dc7e16 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -26,7 +26,7 @@
 			sgsn_main.c sgsn_vty.c sgsn_libgtp.c \
 			gprs_llc.c gprs_llc_parse.c gprs_llc_vty.c crc24.c \
 			sgsn_ctrl.c sgsn_auth.c gprs_subscriber.c \
-			osmo_gsup_messages.c gprs_utils.c gprs_gsup_client.c \
+			gprs_utils.c gprs_gsup_client.c \
 			sgsn_cdr.c sgsn_ares.c \
 			oap.c oap_messages.c
 osmo_sgsn_LDADD = 	\