kbuild: rpm-pkg: keep spec file until make mrproper

kbuild: rpm-pkg: keep spec file until make mrproper

commit af60e207087975d069858741c44ed4f450330ac4 upstream.

If build fails during (bin)rpm-pkg, the spec file is not cleaned by
anyone until the next successful build of the package.

We do not have to immediately delete the spec file in case somebody
may want to take a look at it.  Instead, make them ignored by git,
and cleaned up by make mrproper.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>