Fixed Qt version in qmlobserver

QT_VERSION_STR is being read from Qt's source directory
and may differ from the version of the build being used

See also printVersion() in src/app/main.cpp

Change-Id: Ib740c07136e1549b0cbecc8258889688763b0b36
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>