sout std: use raw mux for .mp3 extension (fix #3910)

(cherry picked from commit 53a9e90cea0976140f2f7f8e07c4cdc20bb0fc17)

modules/stream_out/standard.c

@@ -249,6 +249,7 @@ static int Open( vlc_object_t *p_this )
             { "avi", "avi" },
             { "ogg", "ogg" },
             { "ogm", "ogg" },
+            { "mp3", "raw" },
             { "mp4", "mp4" },
             { "mov", "mov" },
             { "moov","mov" },