shill: Add C++11 override specifier to destructors of derived classes.

BUG=None
TEST=`USE=wimax FEATURES=test emerge-$BOARD platform2`

Change-Id: I88d4e29165869b26924cccb94ac940dc8961c312
Reviewed-on: https://chromium-review.googlesource.com/212267
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/vpn_service.h b/vpn_service.h
index 5f9a0b1..02c0870 100644
--- a/vpn_service.h
+++ b/vpn_service.h
@@ -25,7 +25,7 @@
              Metrics *metrics,
              Manager *manager,
              VPNDriver *driver);  // Takes ownership of |driver|.
-  virtual ~VPNService();
+  ~VPNService() override;
 
   // Inherited from Service.
   virtual void Connect(Error *error, const char *reason);