[eval] fix some getters in externs being public

(default visibility is flipped in extern classes, so these were showing up in completion)

std/eval/_std/StringBuf.hx

@@ -24,7 +24,7 @@
 extern class StringBuf {
 	public var length(get,never) : Int;
 	public function new():Void;
-	function get_length() : Int;
+	private function get_length() : Int;
 	public function add<T>( x : T ) : Void;
 	public function addChar( c : Int ) : Void;
 	public function addSub( s : String, pos : Int, ?len : Int) : Void;

std/eval/_std/haxe/io/BytesBuffer.hx

@@ -25,7 +25,7 @@ package haxe.io;
 extern class BytesBuffer {
 	public var length(get, never):Int;
 	public function new():Void;
-	function get_length():Int;
+	private function get_length():Int;
 	public function addByte(byte:Int):Void;
 	public function add(src:Bytes):Void;
 	public function addString(v:String,?encoding:Encoding):Void;