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/mock_external_task.h b/mock_external_task.h
index 8a3b979..0da1c97 100644
--- a/mock_external_task.h
+++ b/mock_external_task.h
@@ -21,7 +21,7 @@
                    GLib *glib,
                    const base::WeakPtr<RPCTaskDelegate> &task_delegate,
                    const base::Callback<void(pid_t, int)> &death_callback);
-  virtual ~MockExternalTask();
+  ~MockExternalTask() override;
 
   MOCK_METHOD5(Start,
                bool(const base::FilePath &file,