Silence a GCC 8 warning in qquickgeocoordinateanimation

qtlocation/src/imports/positioning/qquickgeocoordinateanimation.cpp:278:106: error: cast between incompatible function types from ‘QVariant (*)(const QGeoCoordinate&, const QGeoCoordinate&, qreal)’ {aka ‘QVariant (*)(const QGeoCoordinate&, const QGeoCoordinate&, double)’} to ‘QVariantAnimation::Interpolator’ {aka ‘QVariant (*)(const void*, const void*, double)’} [-Werror=cast-function-type]
         d->interpolator = reinterpret_cast<QVariantAnimation::Interpolator>(&q_coordinateWestInterpolator);

Change-Id: If8728c01782177aeeb0f64a8637093d0c4c4ee2d
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>