Rename gprs_gb.[c,h] -> gprs_ns.[c,h]

All remaining code in that file is NS protocol related, hence let's
rename it so that we end up with one file per protocol in the Gb stack.

Change-Id: I8312c8a70d60cab48764950c5b57ca02964e9db2
diff --git a/src/sgsn/sgsn_vty.c b/src/sgsn/sgsn_vty.c
index db76c35..7a94f79 100644
--- a/src/sgsn/sgsn_vty.c
+++ b/src/sgsn/sgsn_vty.c
@@ -35,7 +35,7 @@
 #include <osmocom/sgsn/debug.h>
 #include <osmocom/sgsn/sgsn.h>
 #include <osmocom/gprs/gprs_ns2.h>
-#include <osmocom/sgsn/gprs_gb.h>
+#include <osmocom/sgsn/gprs_ns.h>
 #include <osmocom/sgsn/gprs_gmm.h>
 #include <osmocom/sgsn/gprs_bssgp.h>
 #include <osmocom/sgsn/gprs_sgsn.h>