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/private_header.hpp b/extensions/openpower-pels/private_header.hpp
index 224142c..dbdaca6 100644
--- a/extensions/openpower-pels/private_header.hpp
+++ b/extensions/openpower-pels/private_header.hpp
@@ -38,7 +38,7 @@
 {
   public:
     PrivateHeader() = delete;
-    ~PrivateHeader() = default;
+    ~PrivateHeader() override = default;
     PrivateHeader(const PrivateHeader&) = default;
     PrivateHeader& operator=(const PrivateHeader&) = default;
     PrivateHeader(PrivateHeader&&) = default;