Split out GPRS SM layer into its own file

Change-Id: Ie61d22e7868af6de73cdf9c731f07130b282599d
diff --git a/src/sgsn/gprs_sgsn.c b/src/sgsn/gprs_sgsn.c
index 1c23d06..189f7bb 100644
--- a/src/sgsn/gprs_sgsn.c
+++ b/src/sgsn/gprs_sgsn.c
@@ -39,6 +39,7 @@
 #include <osmocom/sgsn/gprs_sgsn.h>
 #include <osmocom/sgsn/sgsn.h>
 #include <osmocom/sgsn/gprs_gmm.h>
+#include <osmocom/sgsn/gprs_sm.h>
 #include <osmocom/sgsn/gprs_utils.h>
 #include <osmocom/sgsn/signal.h>
 #include <osmocom/sgsn/gprs_gmm_attach.h>