PEL: Add 'override' to derived destructors

cppcheck complains that it is overriding a virtual function in the base
class but isn't marked with override.

Change-Id: I4e03f0524987427e3742672d27b86c71943fab5b
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/extensions/openpower-pels/failing_mtms.hpp b/extensions/openpower-pels/failing_mtms.hpp
index b14f1ef..7f728c3 100644
--- a/extensions/openpower-pels/failing_mtms.hpp
+++ b/extensions/openpower-pels/failing_mtms.hpp
@@ -21,7 +21,7 @@
 {
   public:
     FailingMTMS() = delete;
-    ~FailingMTMS() = default;
+    ~FailingMTMS() override = default;
     FailingMTMS(const FailingMTMS&) = default;
     FailingMTMS& operator=(const FailingMTMS&) = default;
     FailingMTMS(FailingMTMS&&) = default;