Fix cast-function-type gcc 8 warning

Gcc 8 now warns against casting functions taking references to
functions taking pointers:

warning: cast between incompatible function types from ‘QVariant (*)(const int&, const int&, double)’ to
‘QVariant (*)(const void*, const void*, double)’ [-Werror=cast-function-type]

Suppressed by casting over void(*)().

Change-Id: I48bee53253b8e87110e8a49efa761fc2add89c5a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 0ffc3980

)
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/corelib/animation/qvariantanimation.h

@@ -113,7 +113,7 @@ private:
 
 template <typename T>
 void qRegisterAnimationInterpolator(QVariant (*func)(const T &from, const T &to, qreal progress)) {
-    QVariantAnimation::registerInterpolator(reinterpret_cast<QVariantAnimation::Interpolator>(func), qMetaTypeId<T>());
+    QVariantAnimation::registerInterpolator(reinterpret_cast<QVariantAnimation::Interpolator>(reinterpret_cast<void(*)()>(func)), qMetaTypeId<T>());
 }
 
 #endif //QT_NO_ANIMATION