Split out GPRS SM layer into its own file

Change-Id: Ie61d22e7868af6de73cdf9c731f07130b282599d
diff --git a/src/sgsn/gprs_gb.c b/src/sgsn/gprs_gb.c
index 65342cf..2d9c25a 100644
--- a/src/sgsn/gprs_gb.c
+++ b/src/sgsn/gprs_gb.c
@@ -31,6 +31,7 @@
 #include <osmocom/sgsn/gprs_mm_state_gb_fsm.h>
 #include <osmocom/sgsn/gprs_sgsn.h>
 #include <osmocom/sgsn/gprs_gmm.h>
+#include <osmocom/sgsn/gprs_sm.h>
 #include <osmocom/sgsn/debug.h>
 
 /* Main entry point for incoming 04.08 GPRS messages from Gb */