d/rules: Properly disable media backends

meson_options.txt lists media-ffmpeg and media-gstreamer as separate feature toggles. This fixes a problem with dh_missing failing the package build if media backend dependencies have been found by meson, like:

 dh_missing: warning: usr/lib/aarch64-linux-gnu/gtk-4.0/4.0.0/media/libmedia-gstreamer.so exists in debian/install/deb but is not installed to anywhere

/cc @3v1n0-guest