Squish: Do not hook into subprocesses

With hooking, qmake is not being executed correctly on Linux.

Change-Id: I4dfd633485256bc351f55263c563987aa5a503a3
Reviewed-on: http://codereview.qt-project.org/5751


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Christian Stenger <christian.stenger@nokia.com>