Update debian/control:

- bump Standards-Version to 4.5.1 (was 4.5.0): no changes required
- bump debhelper-compat version to 13 (was 12)
- remove build dependencies from libopus-dev, libspeex-dev, libtheora-dev
  and libvpx-dev
- add build dependency from libgstreamer1.0-dev
- add extra dependencies from gstreamer1.0-plugins-base,
  gstreamer1.0-plugins-good and gstreamer1.0-plugins-ugly to binary package
  libqxmpp3