Move llc->MM/SM Gb specific glue code to its own file

Now that we have RANAP/Iu handling specificities in its own file, let's
have also Gb specific glue code for messages coming from llc up to MM/SM
layer in its own file. This way same entry points in gprs_gmm.c are used
by Gb and Iu: gsm0408_rcv_gmm() (for MM) and gsm0408_rcv_gsm() (for SM).

Change-Id: Iaf57922a0970c1d03f6f1d6337d27ae3d4aaf32c
diff --git a/include/osmocom/sgsn/Makefile.am b/include/osmocom/sgsn/Makefile.am
index b4cd5db..b91c9f1 100644
--- a/include/osmocom/sgsn/Makefile.am
+++ b/include/osmocom/sgsn/Makefile.am
@@ -3,6 +3,7 @@
 	crc24.h \
 	debug.h \
 	gb_proxy.h \
+	gprs_gb.h \
 	gprs_gb_parse.h \
 	gprs_gmm.h \
 	gprs_gmm_attach.h \