kbuild: clean Modules.symvers in external module dirs

At the moment, running `make clean` in an external module directory does a
nice job of cleaning up with one exception: it leaves behind Modules.symvers.
Attached patch adds this file to the clean list for external modules.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>