kbuild: fix mkspec to cleanup RPM_BUILD_ROOT

The contents of the %clean section in mkspec is currently commented
out leaving RPM_BUILD_ROOT and its contents on the build machine.
This patch removes it once the rpm build process is complete.

Signed-off-by: Josh Hunt <josh@scalex86.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>