Add missing <functional> include

Required by g++8 and clang-8

src/language/BuiltinFunctionEmbedder.hpp

 #ifndef BUILTIN_FUNCTION_EMBEDDER_HPP
 #define BUILTIN_FUNCTION_EMBEDDER_HPP
 
-#include <memory>
-#include <vector>
-
 #include <ASTNodeDataType.hpp>
 #include <DataVariant.hpp>
 
 #include <DataHandler.hpp>
 
+#include <memory>
+#include <vector>
+#include <functional>
+
 template <typename T>
 inline ASTNodeDataType ast_node_data_type_from = ASTNodeDataType::undefined_t;