Back in 5dddf134 I moved the build of
appstream-glib from the Autotools to Meson, trying to keep the same
explicit and default build options.

That means I set -Dbuilder=false and -Drpm=true, among other things.

One thing I didn't notice at the time is that -Dbuilder=false implied
-Drpm=false as well. As a result appstream-glib would build just fine,
even when the RPM libraries weren't present in the Sdk.

However, nowadays appstream-glib doesn't build any more in that specific
combination: https://github.com/hughsie/appstream-glib/pull/294

Since the rpm support always ended disabled anyway, let's explicitly
disable the option.