posix: time.h: Add workaround for outdated newlib used by Xtensa

Unfortunately, Zephyr SDK 0.10.0 ships with outdated Newlib 2.0.0
(from 2015 or earlier) which lacks sys/_timespec.h header, requiring
ugly workaround of defining struct timespec inline (the whole idea
was to standardize on sys/_timespec.h header for different libc's).

This is similar to earlier workaround for struct timeval definition
introduced in a6aee9b4. Zephyr SDK ticket for this issue
is https://github.com/zephyrproject-rtos/sdk-ng/issues/64

, and it
will ve possible to remove both workarounds when Xtensa toolchain
will be upgraded to newlib version consistent with other
architectures.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>