Create new specific file for BSSGP code

Right now there's no much code there since the related code is totally
entangled with the LLC one.
This will eventually change in the future when we switch to use
libosmo-gprs.
Hence, this commit is a preparation to have already some place to put
new BSSGP specific code in the future.

Change-Id: I816396ab5ccb86032bbc21b41a959934a7768780
diff --git a/src/sgsn/sgsn_vty.c b/src/sgsn/sgsn_vty.c
index 0929f1f..db76c35 100644
--- a/src/sgsn/sgsn_vty.c
+++ b/src/sgsn/sgsn_vty.c
@@ -37,6 +37,7 @@
 #include <osmocom/gprs/gprs_ns2.h>
 #include <osmocom/sgsn/gprs_gb.h>
 #include <osmocom/sgsn/gprs_gmm.h>
+#include <osmocom/sgsn/gprs_bssgp.h>
 #include <osmocom/sgsn/gprs_sgsn.h>
 #include <osmocom/sgsn/gtp_mme.h>
 #include <osmocom/sgsn/vty.h>
@@ -1284,7 +1285,7 @@
 		return CMD_WARNING;
 	}
 
-	gprs_gb_page_ps_ra(mm);
+	sgsn_bssgp_page_ps_ra(mm);
 	return CMD_SUCCESS;
 }