Ensure 'make clean' descends to u2mfn/

Makefile

@@ -41,5 +41,6 @@ clean:
 	(cd dom0/restore && make clean)
 	(cd dom0/qmemman && make clean)
 	(cd common && make clean)
+	(cd u2mfn && make clean)
 	make -C qrexec clean
 	make -C vchan clean