Added 'this' keyword to enable builds to succeed

This fixes build errors like the following:

OpenCLStencilFactory.cpp: In instantiation of 'Stencil<T>* OpenCLStencilFactory<T>::BuildStencil(const OptionParser&) [with T = double]':
Stencil2Dmain.cpp:466:1:   required from here
OpenCLStencilFactory.cpp:20:5: error: 'ExtractOptions' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
OpenCLStencilFactory.cpp:20:5: note: declarations in dependent base 'CommonOpenCLStencilFactory<double>' are not found by unqualified lookup
OpenCLStencilFactory.cpp:20:5: note: use 'this->ExtractOptions' instead