Fix compilation of qeasingcurve autotest on Windows.

MSVC cannot decide on an overload for pow().

Change-Id: Ied3fbc0de403774d9f85738852cf671ce42cd1c1
Reviewed-by: Jason McDonald <jason.mcdonald@nokia.com>

tests/auto/corelib/tools/qeasingcurve/tst_qeasingcurve.cpp

@@ -753,7 +753,7 @@ void tst_QEasingCurve::testCbrtFloat()
         const float t_cubic = t * t * t;
         t = t * (t_cubic + f + f) / (t_cubic + t_cubic + f);
 
-        float expected = pow(f, 1.0/3.0);
+        float expected = pow(f, float(1.0/3.0));
 
         const qreal error = qAbs(expected - t);