Fix Clang warning of constant-false comparison

qstringvaluefns.cpp:224:23: error: comparison of constant -1 with expression of type 'QString::NormalizationForm' is always false [-Werror,-Wtautological-constant-out-of-range-compare]

Change-Id: Ia0dd98f72521709712f5ed11ecf19a48acaf1998
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/xmlpatterns/functions/qstringvaluefns.cpp

@@ -221,7 +221,7 @@ Expression::Ptr NormalizeUnicodeFN::compress(const StaticContext::Ptr &context)
         m_normForm = static_cast<QString::NormalizationForm>(
                 determineNormalizationForm(context->dynamicContext()));
 
-        if(m_normForm == -1)
+        if (int(m_normForm) == -1)
             return m_operands.first();
 
         /* Remove the operand since we don't need it anymore. */