Make ui_message move constructible

There is no particular reason why it shouldn't be

Author: hannes-steffenhagen-diffblue

src/util/ui_message.h

@@ -24,6 +24,7 @@ class ui_message_handlert : public message_handlert
   ui_message_handlert(const class cmdlinet &, const std::string &program);
 
   explicit ui_message_handlert(message_handlert &);
+  ui_message_handlert(ui_message_handlert &&) = default;
 
   virtual ~ui_message_handlert();