asio: object_server: add virtual destructors

Clean up warnings such as:

    ../sdbusplus/asio/object_server.hpp:189:7: error: base class
    ‘class sdbusplus::asio::callback’ has accessible non-virtual
    destructor [-Werror=non-virtual-dtor]

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Idd0ab70549dd47bf52b02362f1bfc20824109680
diff --git a/sdbusplus/asio/object_server.hpp b/sdbusplus/asio/object_server.hpp
index aba453f..dde17e9 100644
--- a/sdbusplus/asio/object_server.hpp
+++ b/sdbusplus/asio/object_server.hpp
@@ -30,6 +30,7 @@
 class callback
 {
   public:
+    virtual ~callback() = default;
     virtual int call(message::message& m) = 0;
 };
 
@@ -43,6 +44,7 @@
 class callback_set
 {
   public:
+    virtual ~callback_set() = default;
     virtual SetPropertyReturnValue call(message::message& m) = 0;
     virtual SetPropertyReturnValue set(const boost::any& value) = 0;
 };