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/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index b5a61b1..bdbad19 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -23,7 +23,8 @@
 			sgsn_main.c sgsn_vty.c sgsn_libgtp.c \
 			gprs_llc.c gprs_llc_parse.c gprs_llc_vty.c crc24.c \
 			sgsn_ctrl.c sgsn_auth.c gprs_subscriber.c \
-			gprs_gsup_messages.c gprs_utils.c gprs_gsup_client.c
+			gprs_gsup_messages.c gprs_utils.c gprs_gsup_client.c \
+			gsm_04_08_gprs.c
 osmo_sgsn_LDADD = 	\
 			$(top_builddir)/src/libcommon/libcommon.a \
 			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS)