MAINTAINERS: Add security/Kconfig.hardening to hardening section

MAINTAINERS: Add security/Kconfig.hardening to hardening section

When running get_maintainer.pl on security/Kconfig.hardening, only the
security subsystem folks show up, even though they have never taken
patches to this file:

  $ scripts/get_maintainer.pl security/Kconfig.hardening
  Paul Moore <...> (supporter:SECURITY SUBSYSTEM)
  James Morris <...> (supporter:SECURITY SUBSYSTEM)
  "Serge E. Hallyn" <...> (supporter:SECURITY SUBSYSTEM)
  linux-security-module@vger.kernel.org (open list:SECURITY SUBSYSTEM)
  linux-kernel@vger.kernel.org (open list)

  $ git log --format=%cn --no-merges security/Kconfig.hardening | sort | uniq -c
        3 Andrew Morton
        1 Greg Kroah-Hartman
       18 Kees Cook
        2 Linus Torvald

Add it to the hardening section so that the KSPP folks are also shown,
which matches reality over who should comment on and take said patches
if necessary.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20240928-maintainers-security-kconfig-hardening-v1-1-c8c64071cc02@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>