api: fix warning

Author: sekrit-twc

src/zimg/api/zimg++.hpp

@@ -330,6 +330,6 @@ class SubGraph {
 };
 
 } // namespace zimgxx
-#endif ZIMG_GRAPHENGINE_API // ZIMG_GRAPHENGINE_API
+#endif // ZIMG_GRAPHENGINE_API
 
 #endif // ZIMGPLUSPLUS_HPP_