*: add a couple of <functional> because of equal_to

src/entt/graph/flow.hpp

@@ -3,6 +3,7 @@
 
 #include <algorithm>
 #include <cstddef>
+#include <functional>
 #include <iterator>
 #include <memory>
 #include <type_traits>

src/entt/signal/dispatcher.hpp

@@ -2,6 +2,7 @@
 #define ENTT_SIGNAL_DISPATCHER_HPP
 
 #include <cstddef>
+#include <functional>
 #include <memory>
 #include <type_traits>
 #include <utility>