misc: clean copies of vmmon.o and vmnet.o in top level makefile

Makefiles in *-only copy the object file module is created from into the
parent directory but do not delete them on "make clean". Clean these files
up in "clean" target of top level makefile.

Makefile

@@ -43,6 +43,7 @@ install: retiredcheck $(MODFILES)
 	if test -z "$(DESTDIR)"; then $(DEPMOD) -a $(VM_UNAME); fi
 
 clean: $(SUBDIRS)
+	rm -f *.o
 
 tarballs: $(TARBALLS)