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>