Fix an issue found by cppcheck

cppcheck finds an issue:

 [sdbusplus/asio/object_server.hpp:46] ->
 [sdbusplus/asio/object_server.hpp:315]: (style) The function 'set'
 overrides a function in a base class but is not marked with a
 'override' specifier.

Fix it by adding the override keyword correctly.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: Ie032229874bb416f7c6ec142d9bf38f8ffcff877
diff --git a/sdbusplus/asio/object_server.hpp b/sdbusplus/asio/object_server.hpp
index 5ec0a58..91b58ee 100644
--- a/sdbusplus/asio/object_server.hpp
+++ b/sdbusplus/asio/object_server.hpp
@@ -312,7 +312,7 @@
         }
         return SetPropertyReturnValue::fail;
     }
-    SetPropertyReturnValue set(const boost::any& value)
+    SetPropertyReturnValue set(const boost::any& value) override
     {
         auto oldValue = *value_;
         if (func_(boost::any_cast<PropertyType>(value), *value_))