posix: struct timespec: Move definition to sys/_timespec.h

POSIX subsys defines struct timespec in <time.h> (as POSIX public
API requires), but newlib defines in in sys/_timespec.h, which
inevitably leads to inclusion order and redifinition conflicts.
Follow newlib way and define it in single place, sys/_timespec.h,
which belongs to libc namespace. Thus, we move current definition
to minimal libc, and will use either minlibc's or newlib's
definition, instead of trying to redefine it.

This is similar to the introduction of sys/_timeval.h done earlier.

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