drivers uart_native_ptty: Set standard source macro appropriately

This file uses several functions which are extensions to the the
std C library. Let's explicity select one of the extensions
which includes it instead of relaying on somebody having
set it for this file somewhere else.

(cherry picked from commit eb38e8d)

Original-Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
GitOrigin-RevId: eb38e8d
Change-Id: I0c960c4bf4cd0afaf4c0a820ccd28fb17f3884cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5242623
Tested-by: Al Semjonovs <asemjonovs@google.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>
Commit-Queue: Al Semjonovs <asemjonovs@google.com>
Tested-by: Tristan Honscheid <honscheid@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Tristan Honscheid <honscheid@google.com>

drivers/serial/uart_native_ptty_bottom.c

@@ -5,6 +5,12 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#undef _XOPEN_SOURCE
+/* Note: This is used only for interaction with the host C library, and is therefore exempt of
+ * coding guidelines rule A.4&5 which applies to the embedded code using embedded libraries
+ */
+#define _XOPEN_SOURCE 600
+
 #include <stdbool.h>
 #include <errno.h>
 #include <stddef.h>