Fix for compiling with clang with c++11.

Change-Id: Ibb1e3290edbb2bdc68bb8164be6c484e49b9c69e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qregexp.cpp

@@ -3812,7 +3812,7 @@ struct QRegExpPrivate
 };
 
 #if !defined(QT_NO_REGEXP_OPTIM)
-uint qHash(const QRegExpEngineKey &key, uint seed)
+uint qHash(const QRegExpEngineKey &key, uint seed = 0) Q_DECL_NOTHROW
 {
     return qHash(key.pattern, seed);
 }