bts: constify arg in func bts_ms_store()

Change-Id: I4cc8c4fc075cdd07e689511df8f1c267e5360014
diff --git a/src/bts.cpp b/src/bts.cpp
index bcf4d1b..4fe529e 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -1066,7 +1066,7 @@
 	return ms;
 }
 
-struct GprsMsStorage *bts_ms_store(struct gprs_rlcmac_bts *bts)
+struct GprsMsStorage *bts_ms_store(const struct gprs_rlcmac_bts *bts)
 {
 	return bts->ms_store;
 }