sparc: explicitly include sched.h to get task_thread_info declaration

This was caused by commit 16559ae48c76 ("kgdb: remove #include
<linux/serial_8250.h> from kgdb.h") from the tty tree.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>