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>

src/gui/text/qtextengine.cpp

@@ -2762,6 +2762,7 @@ struct QScriptItemComparator {
 #if defined(Q_CC_MSVC) && _MSC_VER < 1600
 //The STL implementation of MSVC 2008 requires the definition
     bool operator()(const QScriptItem &a, int p) { return a.position < p; }
+    bool operator()(const QScriptItem &a, const QScriptItem &b) { return a.position < b.position; }
 #endif
 };
 }