Fix compilation on QNX

Add Q_DECL_NOEXCEPT to 'bool seed()' and 'quint32 nextMask()'

  qdefaultmaskgenerator_p.cpp:93:34: error: declaration of 'virtual bool QDefaultMaskGenerator::seed()' has a different exception specifier
  qdefaultmaskgenerator_p.h:69:10: error: from previous declaration 'virtual bool QDefaultMaskGenerator::seed() noexcept (true)'
  qdefaultmaskgenerator_p.cpp:104:41: error: declaration of 'virtual quint32 QDefaultMaskGenerator::nextMask()' has a different exception specifier
  qdefaultmaskgenerator_p.h:70:13: error: from previous declaration 'virtual quint32 QDefaultMaskGenerator::nextMask() noexcept (true)'

Change-Id: Iae49a060fa62a33f629cadc1a0100653a12432f7
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>