builtin,os: fix unused return C warnings for real (#9814)

vlib/os/os_c.v

@@ -740,10 +740,11 @@ pub fn is_link(path string) bool {
 
 // chdir changes the current working directory to the new directory in `path`.
 pub fn chdir(path string) {
+	mut n := 0
 	$if windows {
 		C._wchdir(path.to_wide())
 	} $else {
-		C.chdir(&char(path.str))
+		n = C.chdir(&char(path.str))
 	}
 }