[GPRS] Rename gsm_04_08_gprs.c to gprs_gmm.c

GMM refers to GPRS Mobility Management.
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 88c358a..6d63145 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -5,7 +5,7 @@
 sbin_PROGRAMS = osmo-gbproxy osmo-sgsn
 noinst_LIBRARIES = libsgsn.a
 
-libsgsn_a_SOURCES = gprs_ns.c gprs_bssgp.c gprs_llc.c gsm_04_08_gprs.c \
+libsgsn_a_SOURCES = gprs_ns.c gprs_bssgp.c gprs_llc.c gprs_gmm.c \
 		crc24.c gprs_sgsn.c gprs_bssgp_util.c
 
 osmo_gbproxy_SOURCES = gb_proxy.c gb_proxy_main.c gb_proxy_vty.c \
diff --git a/openbsc/src/gprs/gsm_04_08_gprs.c b/openbsc/src/gprs/gprs_gmm.c
similarity index 100%
rename from openbsc/src/gprs/gsm_04_08_gprs.c
rename to openbsc/src/gprs/gprs_gmm.c