[lua] add missing ffi cdef signature

std/lua/Ffi.hx

@@ -30,6 +30,7 @@ extern class Ffi {
 	public function new(type : String, arg : Dynamic);
 
 	// Declaring and accessing external symbols
+	public static function cdef(def : String) : Void;
 	public static var C : Dynamic;
 	public static function gc(cdata : Dynamic, finalizer : Function) : Void;
 	public static function load (name : String, ?global : Bool) : Dynamic;