BUILD: fd: Make sure to include sched.h

BUILD: fd: Make sure to include sched.h

Commit 5e9662ddd63a730235deec27489c7e8897fea21d made it so that instead
of checking for the existence of CLONE_FILES, we'd define
HA_HAVE_UNSHARE if file unsharing is available. To determine that, it
checks for the existence of CLONE_FILES, however without including
sched.h, it will never be defined, and file unsharing will never be
used.
Fix that by properly including sched.h.