gprs: Move protocol value_strings to gsm_04_08_gprs.c

Currently the mapping between GSM 04.08 (GPRS) protocol specific
numbers and their textual description was put into gprs_gmm.c and not
exported.

This commit moves the mappings to a new file gsm_04_08_gprs.c,
renames some of them, and exports them via gsm_04_08_gprs.h.

The following identifiers are renamed to match the corresponding type
names:

  - gmm_cause_names -> gsm48_gmm_cause_names
  - gsm_cause_names -> gsm48_gsm_cause_names

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/Makefile.am b/openbsc/tests/sgsn/Makefile.am
index 70e0924..672b811 100644
--- a/openbsc/tests/sgsn/Makefile.am
+++ b/openbsc/tests/sgsn/Makefile.am
@@ -25,6 +25,7 @@
 	$(top_builddir)/src/gprs/gprs_gsup_client.o \
 	$(top_builddir)/src/gprs/gprs_utils.o \
 	$(top_builddir)/src/gprs/gprs_subscriber.o \
+	$(top_builddir)/src/gprs/gsm_04_08_gprs.o \
 	$(top_builddir)/src/libcommon/libcommon.a \
 	$(LIBOSMOABIS_LIBS) \
 	$(LIBOSMOCORE_LIBS) \