Qtbase submodule ignored for synqt submodule version detection.

Submodule versions are detected during src build, and qtbase is a
special case and has to be treated as such.

Change-Id: I6234e5349a9c7a9943c5fdb91189f4aa26195c5f
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>