posix: Define _POSIX_C_SOURCE to gain access to POSIX functions

options/fs.c and options/timer.c both use POSIX-only functions. To ensure
those symbols are visible from the underlying C library, define
_POSIX_C_SOURCE.

Signed-off-by: Keith Packard <keithp@keithp.com>

lib/posix/options/fs.c

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
 #include <errno.h>
 #include <zephyr/kernel.h>
 #include <limits.h>

lib/posix/options/timer.c

@@ -4,6 +4,8 @@
  *
  * SPDX-License-Identifier: Apache-2.0
  */
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
 #include <errno.h>
 
 #include <zephyr/kernel.h>