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/user_header.hpp b/extensions/openpower-pels/user_header.hpp
index 4d3e7cf..d212831 100644
--- a/extensions/openpower-pels/user_header.hpp
+++ b/extensions/openpower-pels/user_header.hpp
@@ -31,7 +31,7 @@
 {
   public:
     UserHeader() = delete;
-    ~UserHeader() = default;
+    ~UserHeader() override = default;
     UserHeader(const UserHeader&) = default;
     UserHeader& operator=(const UserHeader&) = default;
     UserHeader(UserHeader&&) = default;