drivers flash_simulator: 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 318f824)

Original-Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
GitOrigin-RevId: 318f824
Change-Id: Ib99028c24ec84fba43c72638299cc7ec6bff14c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5242621
Commit-Queue: Al Semjonovs <asemjonovs@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Tristan Honscheid <honscheid@google.com>
Tested-by: Al Semjonovs <asemjonovs@google.com>
Tested-by: Tristan Honscheid <honscheid@google.com>
Reviewed-by: Al Semjonovs <asemjonovs@google.com>

drivers/flash/flash_simulator_native.c

@@ -9,6 +9,12 @@
  * native simulator runner/host context, and not in Zephyr/embedded context.
  */
 
+#undef _POSIX_C_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 _POSIX_C_SOURCE 200809L
+
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/stat.h>