sdp: cast away const in call to g_free

MSVC warns about the const here. It's safe to cast away.

https://bugzilla.gnome.org/show_bug.cgi?id=774293

gst-libs/gst/sdp/gstsdpmessage.c

@@ -361,7 +361,7 @@ gst_sdp_message_copy (const GstSDPMessage * msg, GstSDPMessage ** copy)
 
     gst_sdp_message_add_time (cp, time->start, time->stop, repeat);
 
-    g_free (repeat);
+    g_free ((gchar **) repeat);
   }
 
   len = gst_sdp_message_zones_len (msg);