include: posix: time: Make possible to include with CONFIG_ARCH_POSIX

While CONFIG_ARCH_POSIX in general isn't compatible with Zephyr POSIX
subsys (because CONFIG_ARCH_POSIX itself is implemented on top of
POSIX, so there're obvious conflicts), apply workaround to allow to
at least use clock_gettime() and clock_settime() functions.

This change is grounded in upcoming support for date manipulation
commands for Zephyr shell, which are implemented using functions
above. There's no guarantee that CONFIG_ARCH_POSIX and Zephyr POSIX
subsys will coexist for any other usecase. (But the change is
relatively clean and is definitely in the right direction of
prototyping ways of such a coexistance.)

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
(cherry picked from commit 4a05644bce81f4792c018964d29c1158f1982d1e)