Fix warning when compiling x86 with SSE2 but no AVX2

qstring.cpp:595:13: error: unused variable ‘nullmask’ [-Werror=unused-variable]

Change-Id: I1cc7601489634e96833cfffd1456474a529a79ed
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Erik Verbruggen <erik.verbruggen@qt.io>

src/corelib/tools/qstring.cpp

@@ -594,6 +594,7 @@ static int ucstrncmp(const QChar *a, const uchar *c, int l)
     }
 #  else
     // 32-bit, we can't do MOVQ to load 8 bytes
+    Q_UNUSED(nullmask);
     enum { MaxTailLength = 15 };
 #  endif