fix syntax according to https://github.com/HaxeFoundation/haxe/issues/11787

Index: src/optimization/dce.ml
--- src/optimization/dce.ml.orig
+++ src/optimization/dce.ml
@@ -76,7 +76,7 @@ let overrides_extern_field cf c =
 	loop c cf
 
 let is_std_file dce file =
-	List.exists (ExtString.String.starts_with file) dce.std_dirs
+	List.exists (fun s -> ExtString.String.starts_with file s) dce.std_dirs
 
 let keep_metas = [Meta.Keep;Meta.Expose]
 
