MSW Ratsnest: Force marking ratsnest dirty

When updating the status of the ratsnest, we mark its target layer as
dirty to ensure the revised data are used to draw/clear the ratsnest

Fixes: lp:1800301
* https://bugs.launchpad.net/kicad/+bug/1800301

(cherry picked from commit 50eedcf0b3)

pcbnew/tools/pcb_editor_control.cpp

@@ -1104,6 +1104,8 @@ static bool showLocalRatsnest( TOOL_MANAGER* aToolMgr, BOARD* aBoard, const VECT
         }
     }
 
+    aToolMgr->GetView()->MarkTargetDirty( KIGFX::TARGET_OVERLAY );
+
     return true;
 }