qt55: fix qtmultimedia gstreamer support

Without this, configure will say:

  Checking for openal... no
  /tmp/nix-build-qtmultimedia-5.5.0.drv-0/qtmultimedia-opensource-src-5.5.0/qtmultimedia.pro:28:
  Variable GST_VERSION is not defined.
  Checking for resourcepolicy... no

And there may be application runtime errors like

  defaultServiceProvider::requestService(): no service found for - "org.qt-project.qt.camera"

After this fix, configure will say:

  Checking for openal... no
  Checking for gstreamer... yes
  Checking for gstreamer_photography... no
  Checking for gstreamer_encodingprofiles... yes
  Checking for gstreamer_appsrc... yes
  Checking for linux_v4l... yes
  Checking for resourcepolicy... no

And the above runtime error will not appear.

This fix is similar to 449b6028 ("qt5.multimedia: fix gstreamer
support."), except with Qt 5.5 we also need to set GST_VERSION.