examples: Include sys/stat.h for S_I* constants

examples: Include sys/stat.h for S_I* constants

POSIX says that these constants are defined in sys/stat.h. On Linux with
glibc and musl this is also defined in fcntl.h. Other systems define it
only in sys/stat.h, so include that as well. The Linux headers ensure
that the first of sys/stat.h or fcntl.h define the constant, so it's
harmless to include both.

For the latest standard, IEEE Std 1003.1-2017, see
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html
and
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_stat.h.html
for details.

Signed-off-by: Warner Losh <imp@bsdimp.com>