Use the preferences-system-network icon for the network panel

This one has the advantage that it exists in gnome-icon-theme.

panels/network/gnome-network-panel.desktop.in.in

@@ -2,7 +2,7 @@
 _Name=Network
 _Comment=Network settings
 Exec=gnome-control-center network
-Icon=gnome-network-properties
+Icon=preferences-system-network
 Terminal=false
 Type=Application
 StartupNotify=true