make: fix clean rule by removing build file in qom/

Make clean does not clean the 'qom' directory, leaving *.o and *.d files. This
patch fixes this.
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile

@@ -217,6 +217,7 @@ clean:
 	rm -f *.o *.d *.a *.lo $(TOOLS) $(HELPERS-y) qemu-ga TAGS cscope.* *.pod *~ */*~
 	rm -Rf .libs
 	rm -f slirp/*.o slirp/*.d audio/*.o audio/*.d block/*.o block/*.d net/*.o net/*.d fsdev/*.o fsdev/*.d ui/*.o ui/*.d qapi/*.o qapi/*.d qga/*.o qga/*.d
+	rm -f qom/*.o qom/*.d
 	rm -f qemu-img-cmds.h
 	rm -f trace/*.o trace/*.d
 	rm -f trace.c trace.h trace.c-timestamp trace.h-timestamp