os: tag some File methods that take voidptr as unsafe - write_bytes, write_bytes_at (#8985)

examples/wkhtmltopdf.v

@@ -78,7 +78,7 @@ fn main() {
 			println('ERR: $err')
 			return
 		}
-		wrote := file.write_bytes(data, size)
+		wrote := unsafe { file.write_bytes(data, size) }
 		println('write_bytes: $wrote [./google.pdf]')
 		file.flush()
 		file.close()