viewport: event->position() isn't Qt 5.12, tweak version check

According to https://doc.qt.io/qt-5/qwheelevent.html#position the
QWheelEvent::position() method was introduced in Qt 5.14. Adjust
the Qt version check to unbreak builds with Qt 5.12.

pv/views/trace/viewport.cpp

@@ -249,7 +249,7 @@ void Viewport::wheelEvent(QWheelEvent *event)
 				- delta * view_.scale() + view_.offset());
 		} else {
 			// Vertical scrolling is interpreted as zooming in/out
-#if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)
+#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
 			view_.zoom(delta / 120.0, event->position().x());
 #else
 			view_.zoom(delta / 120.0, event->x());