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/tests/sgsn/Makefile.am b/tests/sgsn/Makefile.am
index ae397b3..b035468 100644
--- a/tests/sgsn/Makefile.am
+++ b/tests/sgsn/Makefile.am
@@ -45,6 +45,7 @@
 	$(top_builddir)/src/gprs/gprs_llc_parse.o \
 	$(top_builddir)/src/gprs/gprs_llc.o \
 	$(top_builddir)/src/gprs/crc24.o \
+	$(top_builddir)/src/gprs/gprs_gb.o \
 	$(top_builddir)/src/gprs/gprs_ranap.o \
 	$(top_builddir)/src/gprs/gprs_sndcp.o \
 	$(top_builddir)/src/gprs/gprs_gmm_attach.o \