Fix threading warning discovered with MSVC -Wall

MSVC in -Wall mode creates approximately 6,500 warnings. 5,000+ of which are
from the std library. The one gem was the potential for threading issues with
the initilization of a function scoped static.

This fixes that.

include/chaiscript/dispatchkit/proxy_functions_detail.hpp

@@ -15,6 +15,7 @@
 
 #include <stdexcept>
 #include <vector>
+#include <functional>
 
 namespace chaiscript
 {