VLR: vlr_subscr_{,msisdn_or_}name: const vsub arg

Make the vsub argument of both vlr_subscr_msisdn_or_name()
and vlr_subscr_name() a const.

The LOGVSUBP() macro uses vlr_subscr_name() and will not generate a
warning anymore when used with a const vsub.

Change-Id: If609269191f4df6186d823a2eee14012846328e2
diff --git a/include/osmocom/msc/vlr.h b/include/osmocom/msc/vlr.h
index 029ef31..b328b70 100644
--- a/include/osmocom/msc/vlr.h
+++ b/include/osmocom/msc/vlr.h
@@ -330,8 +330,8 @@
 			uint32_t parent_event_failure);
 
 
-const char *vlr_subscr_name(struct vlr_subscr *vsub);
-const char *vlr_subscr_msisdn_or_name(struct vlr_subscr *vsub);
+const char *vlr_subscr_name(const struct vlr_subscr *vsub);
+const char *vlr_subscr_msisdn_or_name(const struct vlr_subscr *vsub);
 
 #define vlr_subscr_find_by_imsi(vlr, imsi) \
 	_vlr_subscr_find_by_imsi(vlr, imsi, __FILE__, __LINE__)