log_manager: add override to deleteAll

The function 'deleteAll' is intended to override a function
in one of the phosphor-dbus-interfaces classes.  Clang throws
a warning without this being present, so add it.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ibb71c3f2ae296db20369add90b48c69f7af12474
diff --git a/log_manager.hpp b/log_manager.hpp
index ad88f10..b8b250a 100644
--- a/log_manager.hpp
+++ b/log_manager.hpp
@@ -369,7 +369,7 @@
 
     /** @brief Delete all d-bus objects.
      */
-    void deleteAll()
+    void deleteAll() override
     {
         manager.eraseAll();
     }