Stack size increase for x86 QNX systems

Increase the size of the stack for the scene graph thread on x86
QNX systems.  The default stack size (132K) is too small for the
MESA graphics driver shared libraries on these systems.

Task-number: QTBUG-37338
Change-Id: I8b50aece98ef3f530f2202fc89a0e90b0b2e2e72
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>