poxi: aio: include zephyr/toolchain.h for ZRESTRICT

Previously, ZRESTRICT was used in aio.h without also including
zephyr/toolchain.h.

Tests may have compiled because the toolchain header was included by
some other means.

We should explicitly include it to ensure that ZRESTRICT is defined.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

include/zephyr/posix/aio.h

@@ -11,6 +11,8 @@
 #include <sys/types.h>
 #include <time.h>
 
+#include <zephyr/toolchain.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif