Fix QTextEngine build on MSVC2008 debug mode.

This was broken in 7ac6ce21 which removed some unused functions;
but MSVC2008 in debug mode requires this particular operator overload.
I'm re-adding it in an appropriate #ifdef so it doesn't cause "unused"
warnings in other platforms/compilers.

Task-number: QTBUG-33473
Change-Id: I6dfba0fa5f835e848e8a1e4213efb030fb5def3d
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>