Add missing include for move_only_function

 #include <functional>

References:
[1] https://en.cppreference.com/w/cpp/utility/functional/move_only_function

Change-Id: I5b605eab5209a0bed545dfd74166c5eb0a8c3647
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/include/sdbusplus/asio/connection.hpp b/include/sdbusplus/asio/connection.hpp
index 7e7cac0..57504c1 100644
--- a/include/sdbusplus/asio/connection.hpp
+++ b/include/sdbusplus/asio/connection.hpp
@@ -37,6 +37,7 @@
 #include <sdbusplus/utility/type_traits.hpp>
 
 #include <chrono>
+#include <functional>
 #include <string>
 #include <tuple>