Fix silly taking of address when not intended

We meant to pass the boolean itself, noting whether the property
had changed, not the address of the boolean, cast to a bool.

Found by Clang 3.6:
qlayout_widget.cpp:421:147: warning: address of 'this->m_fieldGrowthPolicyChanged' will always evaluate to 'true' [-Wpointer-bool-conversion]

Change-Id: I8a7db0d0cf02b88043f905333e119a8fb799ea53
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>