app-portage/gentoolkit: Create /var/cache/revdep-rebuild in pkg_postinst

app-portage/gentoolkit: Create /var/cache/revdep-rebuild in pkg_postinst

This resolves a QA warning.

QA Notice: This ebuild installs into paths that should be created at runtime.
 To fix, simply do not install into these directories.  Instead, your package
 should create dirs on the fly at runtime as needed via init scripts/etc...

  var/cache
  var/cache/revdep-rebuild
  var/cache/revdep-rebuild/.keep_app-portage_gentoolkit-0

We already install a tmpfiles entry to recreate the directory on boot.

Package-Manager: portage-2.2.28_p57