gprs_ns: Add code for SNS-SIZE and SNS-CONFIG encoding

Modern NS specifications contain a SNS (Sub Network Service) for
negotiating IP/port/weight parameters of NS-over-IP links dynamically.

This patch adds message encoding routines for SNS-CONFIG, SNS-SIZE
and their respective acknowledgements.

Related: OS#3372
Change-Id: I5c47e1c3c10deb89a7470ee2c03adfc174accc93
diff --git a/src/gb/Makefile.am b/src/gb/Makefile.am
index 65b73fe..d074092 100644
--- a/src/gb/Makefile.am
+++ b/src/gb/Makefile.am
@@ -6,7 +6,7 @@
 AM_CFLAGS = -Wall ${GCC_FVISIBILITY_HIDDEN} -fno-strict-aliasing $(TALLOC_CFLAGS)
 
 # FIXME: this should eventually go into a milenage/Makefile.am
-noinst_HEADERS = common_vty.h
+noinst_HEADERS = common_vty.h gb_internal.h
 
 if ENABLE_GB
 lib_LTLIBRARIES = libosmogb.la