scripts: rpm: make clear that mkspec script contains 4.13 feature

A fix was made in the mkspec script that uses a feature, ie. the
OR expression, which requires RPM 4.13. However, the script indicates
another minimum version. Lower versions may have success by using
the --no-deps option as suggested, but feels like bumping the version
to 4.13 is reasonable as it put me on the wrong track at first with
RPM 4.11 on my Centos7 machine.

Fixes: 02a893bc ("kbuild: rpm-pkg: add libelf-devel as alternative for BuildRequires")
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>