kbuild: ensure mrproper removes .old_version

If the final linking of vmlinux fails, the file .old_version are left
behind.  This patch ensures the mrproper target will remove it if
present.

Signed-off-by: Tore Anderson <tore@fud.no>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
1 file changed