posix: options/shm.c needs _POSIX_C_SOURCE

This file uses PATH_MAX, which is defined in limits.h when _POSIX_C_SOURCE
is set.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

lib/posix/options/shm.c

@@ -4,6 +4,9 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+
 #include <string.h>
 #include <stdio.h>