Qt: Set Packet List scrollbar page step only on macOS

Only set the Packet List scrollbar page step to be equal to the
height of the scrollbar when running on macOS. Qt on Linux and
Windows behaves different.

ui/qt/widgets/overlay_scroll_bar.cpp

@@ -145,7 +145,9 @@ void OverlayScrollBar::resizeEvent(QResizeEvent *event)
 
     child_sb_.move(packet_map_width_, 0);
     child_sb_.resize(child_sb_.sizeHint().width(), height());
+#ifdef Q_OS_MAC
     child_sb_.setPageStep(height());
+#endif
 }
 
 void OverlayScrollBar::paintEvent(QPaintEvent *event)