clang-tidy: Enable modernize-use-override check

This check adds override (introduced in C++11) to overridden
virtual functions and removes virtual from those functions as it
is not required.

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I7ce6bf641dabf4d325a29a1b641e58beac6885bd
diff --git a/.clang-tidy b/.clang-tidy
index 92b9cc8..d1d47cd 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -222,6 +222,7 @@
 modernize-use-equals-default,
 modernize-use-equals-delete,
 modernize-use-noexcept,
+modernize-use-override,
 modernize-use-transparent-functors,
 modernize-use-uncaught-exceptions,
 modernize-use-using'