VLR: log subscriber update

* move log helpers to generic header
* log subscriber update

It's handy for troubleshooting issues with subscriber update via GSUP
from HLR.

Change-Id: I1958aeeb3ea99831c7e2c5ee9a6b59834baf4520
diff --git a/src/libmsc/gsm_04_08.c b/src/libmsc/gsm_04_08.c
index 88472e0..3045f71 100644
--- a/src/libmsc/gsm_04_08.c
+++ b/src/libmsc/gsm_04_08.c
@@ -3476,7 +3476,8 @@
 /* VLR informs us that the subscriber data has somehow been modified */
 static void msc_vlr_subscr_update(struct vlr_subscr *subscr)
 {
-	/* FIXME */
+	LOGVSUBP(LOGL_NOTICE, subscr, "VLR: update for IMSI=%s (MSISDN=%s, used=%d)\n",
+		 subscr->imsi, subscr->msisdn, subscr->use_count);
 }
 
 /* VLR informs us that the subscriber has been associated with a conn */