Change deprecated asio function

The io_context.post method has been deprecated for some time.  Move to
the new model, using boost::asio::post.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I946a32e0404809b85c72767aafd783053bf8ee5f
diff --git a/include/sdbusplus/asio/sd_event.hpp b/include/sdbusplus/asio/sd_event.hpp
index 64523a7..0c05daa 100644
--- a/include/sdbusplus/asio/sd_event.hpp
+++ b/include/sdbusplus/asio/sd_event.hpp
@@ -108,7 +108,7 @@
   private:
     void async_run()
     {
-        io.post([this]() { run(); });
+        boost::asio::post(io, [this]() { run(); });
     }
     void async_wait()
     {