Fedora packaging: Add dependency on libvpx

libvpx is needed by the GStreamer element "vp8enc" which is installed by
gstreamer1-plugins-good, which in turn doesn't install the libvpx
dependency, according to
https://groups.google.com/d/msg/stb-tester/ulbGv_4gHdk/x93iqOdQMdYJ