BSSGP: Add functions required for true BSS-side BSSGP implementation

The BSS-side of BSSGP requires quite a number of additional functions
for sending unidirectional messages that a SGSN never sends.

This is a first step into completing the BSSGP implementation and making
it ready to be used from osmo-bts and other BTS-side GPRS
implementations.
diff --git a/openbsc/src/libgb/Makefile.am b/openbsc/src/libgb/Makefile.am
index 8ec1006..cb656cc 100644
--- a/openbsc/src/libgb/Makefile.am
+++ b/openbsc/src/libgb/Makefile.am
@@ -5,5 +5,5 @@
 noinst_LIBRARIES = libgb.a
 
 libgb_a_SOURCES = gprs_ns.c gprs_ns_frgre.c gprs_ns_vty.c \
-		  gprs_bssgp.c gprs_bssgp_util.c gprs_bssgp_vty.c
+		  gprs_bssgp.c gprs_bssgp_util.c gprs_bssgp_vty.c gprs_bssgp_bss.c
 #gprs_llc.c gprs_llc_vty.c crc24.c