Fix missing include

jonnor · jonnor · commit 08b6bba9d3ea · 2017-06-28T20:38:21.000+02:00
Sometimes causing compile errors
diff --git a/include/msgflo.h b/include/msgflo.h
@@ -2,6 +2,8 @@
 
 #include <string>
 #include <memory>
+#include <functional>
+
 #include "json11.hpp"
 
 namespace msgflo {
