serial: sc16is7xx: add proper sched.h include for sched_set_fifo()

Replace incorrect include with the proper one for sched_set_fifo()
declaration.

Fixes: 28d2f209

 ("sched,serial: Convert to sched_set_fifo()")
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20240409154253.3043822-2-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>