tests: fix proto() testcase

Fixes: 4ce69a8 ("fs: implement access(), mkstemp(), file.flush() and proc.flush()")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

tests/custom/03_stdlib/40_proto

@@ -40,6 +40,7 @@ Hello, World!
 	{
 		"error": "function error(...) { [native code] }",
 		"fileno": "function fileno(...) { [native code] }",
+		"flush": "function flush(...) { [native code] }",
 		"close": "function close(...) { [native code] }",
 		"tell": "function tell(...) { [native code] }",
 		"seek": "function seek(...) { [native code] }",