type hint `prev` var to avoid FAnon access (closes #3794)

std/haxe/ds/GenericStack.hx

@@ -115,7 +115,7 @@ class GenericStack<T> {
 		If no matching element is found, false is returned.
 	**/
 	public function remove( v : T ) : Bool {
-		var prev = null;
+		var prev:GenericCell<T> = null;
 		var l = head;
 		while( l != null ) {
 			if( l.elt == v ) {