gsm_subscriber: make function msc_paging_request() static

The function msc_paging_request() is only called from within
gsm_subscriber.c but never from outside. Lets make it static.

Change-Id: I2efc8eac01a4dd8733118067eecf566c13062106
diff --git a/src/libmsc/gsm_subscriber.c b/src/libmsc/gsm_subscriber.c
index c3b8a95..188807e 100644
--- a/src/libmsc/gsm_subscriber.c
+++ b/src/libmsc/gsm_subscriber.c
@@ -109,7 +109,7 @@
 	return 0;
 }
 
-int msc_paging_request(struct vlr_subscr *vsub)
+static int msc_paging_request(struct vlr_subscr *vsub)
 {
 	/* The subscriber was last seen in subscr->lac. Find out which
 	 * BSCs/RNCs are responsible and send them a paging request via open