kbuild: removing .tmp_versions considered harmful

Remove *.mod files but not .tmp_versions for external builds

When "make install" is run as root, .tmp_versions is re-created and
becomes owned by root.  Subsequent "make" run by user fails because
.tmp_versions cannot be removed.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>