libqtav: unstable-2017-03-30 -> 1.12.0

This was actually already mentioned in the comment above version:

  Awaiting upcoming `v1.12.0` release. `v1.11.0` is not supporting
  cmake which is the the reason behind taking an unstable git rev.

So version 1.12.0 has been released in the meantime and it's also needed
by the latest version of digiKam, which is the only package in nixpkgs
that depends on libqtav.

As we're going to bump digiKam, I think bumping this is safe as in the
worst case we can only make things broken which are already broken
(digiKam currently doesn't build).

The fixes for the CMakeLists.txt are now also no longer needed, so we
can safely drop them as well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @jraygauthier