GPRS: constify NS printing routine parameters

Change-Id: I549ab7c26ac1489e01f281594bafe44b0681dc83
diff --git a/src/gb/gprs_ns.c b/src/gb/gprs_ns.c
index 679b59d..a00c8e5 100644
--- a/src/gb/gprs_ns.c
+++ b/src/gb/gprs_ns.c
@@ -1170,7 +1170,7 @@
 	return rc;
 }
 
-const char *gprs_ns_ll_str(struct gprs_nsvc *nsvc)
+const char *gprs_ns_ll_str(const struct gprs_nsvc *nsvc)
 {
 	static char buf[80];