Fix for changed definition of GetCommandString.

* src/gpgex.h (IContextMenu3): Change first arg of GetCommandString to
INT_PTR.
* src/gpgex.cc (GetCommandString): Ditto.  Cast it in the trace call.

With W64 IContextMenu::GetCommandString changed from UINT to UINT_PTR
to accomodate for it being a pointer internally.