man: Prune unneeded <sys/types.h> include in setproctitle(3)

man: Prune unneeded <sys/types.h> include in setproctitle(3)

FreeBSD first declared the function in unistd.h in 2000:
https://cgit.freebsd.org/src/commit/include/unistd.h?id=9feac5c21886
No other #include line was needed to use the function, but the man page
unnecessarily instructed users to include sys/types.h until 2022-11-29:
https://cgit.freebsd.org/src/commit/?id=5b6f0a5012e9bc37dcb32b57b41e2443a46da620

libbsd first declared the function in unistd.h in 2010 with commit
3fed78e5b08f78256e533788b4bcd6502b0949d7 and inherited the bug from
FreeBSD's man page.

No need to do anything for backwards compat.

Signed-off-by: Guillem Jover <guillem@hadrons.org>