QNX: Fix compilation of qdatetime.cpp

Adding std:: to pow() to make it compile for BlackBerry 10.

Change-Id: I9ab3cc626eb2ba872c09df33cdb820b50d075428
Reviewed-by: Wolfgang Bremer <wbremer@blackberry.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>