add sip: when required to server address

[linphone] / mediastreamer2

diff --git a/mediastreamer2 b/mediastreamer2
index 429baf423f2db8673ffa98f7f7d6cbac0c23bde9..df9cac524bbe77240127630fa5d253b19c8fdae7 160000 (submodule)
--- a/mediastreamer2
+++ b/mediastreamer2
@@ -1 +1 @@
-Subproject commit 429baf423f2db8673ffa98f7f7d6cbac0c23bde9
+Subproject commit df9cac524bbe77240127630fa5d253b19c8fdae7