arch: posix: Bugfix in POSIX cheats header

In 8dc69e09 (#10280)
the POSIX API main kconfig option was replaced from
PTHREAD_IPC to POSIX_API.
But the posix_cheats.h header was left using the old option.
This means that some applications/tests which were not
selecting the PTHREAD_IPC API, but using some other Zephyr
POSIX compatible APIs could have trouble when compiled for the
POSIX architecture.

Fixes #13011

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>