restore sending of optional MM info messages

Since commit 2483f1b050496eda7f8707327204251c57212906 the function
gsm48_tx_mm_info() was not called anymore. No MM info messages were
transmitted to phones even if MM info messages were enabled via VTY.

With this commit, we call gsm48_tx_mm_info() after successfully
processing an IMSI ATTACH location update.

Change-Id: Ice5963d84253eb8c803cd2dfa8b25a4db5382827
Related: OS#2850
diff --git a/include/osmocom/msc/vlr.h b/include/osmocom/msc/vlr.h
index 37702a9..96bfa2f 100644
--- a/include/osmocom/msc/vlr.h
+++ b/include/osmocom/msc/vlr.h
@@ -217,6 +217,7 @@
 	/* UTRAN: send Common Id (when auth+ciph are complete) */
 	int (*tx_common_id)(void *msc_conn_ref);
 
+	int (*tx_mm_info)(void *msc_conn_ref);
 
 	/* notify MSC/SGSN that the subscriber data in VLR has been updated */
 	void (*subscr_update)(struct vlr_subscr *vsub);