Move some functions gprs_gb.[c,h] -> gprs_gmm.[c,h]

This leaves only NS protocol related code in gprs_gb.[c,h], which will
be renamed to gprs_ns.[c,h] in a follow up patch.

Change-Id: I3dcbe1d0f75cb91ec8b700e239e2ba16fff030a2
diff --git a/src/sgsn/gprs_sndcp.c b/src/sgsn/gprs_sndcp.c
index 83aec0c..3b9d28a 100644
--- a/src/sgsn/gprs_sndcp.c
+++ b/src/sgsn/gprs_sndcp.c
@@ -41,6 +41,7 @@
 #include <osmocom/sgsn/gprs_sndcp_pcomp.h>
 #include <osmocom/sgsn/gprs_sndcp_dcomp.h>
 #include <osmocom/sgsn/gprs_sndcp_comp.h>
+#include <osmocom/sgsn/gprs_gmm.h>
 
 #define DEBUG_IP_PACKETS 0	/* 0=Disabled, 1=Enabled */