gprs: Add encoder/decoder for the Subscriber Update Protocol

This patch adds functions to encode and decode GSUP messages. This
does not include the layer 1 framing (IPA). The messages so far
supported are: send_auth_info_*, update_location_*,
location_cancellation_*.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index f937362..d45aaee 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -22,7 +22,8 @@
 osmo_sgsn_SOURCES =	gprs_gmm.c gprs_sgsn.c gprs_sndcp.c gprs_sndcp_vty.c \
 			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
+			sgsn_ctrl.c sgsn_auth.c gprs_subscriber.c \
+			gprs_gsup_messages.c gprs_utils.c
 osmo_sgsn_LDADD = 	\
 			$(top_builddir)/src/libcommon/libcommon.a \
 			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS)