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/ppp_device.h b/ppp_device.h
index 560c9cf..391cc93 100644
--- a/ppp_device.h
+++ b/ppp_device.h
@@ -39,7 +39,7 @@
             Manager *manager,
             const std::string &link_name,
             int interface_index);
-  virtual ~PPPDevice();
+  ~PPPDevice() override;
 
   // Set IPConfig for this device, based on the dictionary of
   // configuration strings received from our PPP plugin.