added missing single include guard

ecs/Components.h

@@ -1,3 +1,6 @@
+#ifndef COMPONENTS_H
+#define COMPONENTS_H
+
 #include "utils/HashMap.h"
 #include "utils/List.h"
 #include "utils/Types.h"
@@ -123,4 +126,6 @@ public:
     EntityIteratorAdapter<const Components, const T> entities() const {
         return {*this};
     }
-};
+};
+
+#endif