discovery: use Socket.create_socket_source()

Use Socket.create_socket_source() as it works for both TCP and UDP
sockets, but also does not use GDatagramBased interface, thus should
allow building on systems with older GLib versions.

src/mconnect/discovery.vala

@@ -51,7 +51,7 @@ class Discovery : GLib.Object
 				throw e;
 			}
 
-			var source = (SocketSource) socket.create_source(IOCondition.IN);
+			var source = socket.create_socket_source(IOCondition.IN);
 			source.set_callback((s, c) => {
 					this.incomingPacket();
 					return true;