Fix blkdev.h build errors when BLOCK=n

I see builds failing with:

  CC [M]  drivers/mmc/host/dw_mmc.o
In file included from drivers/mmc/host/dw_mmc.c:15:
include/linux/blkdev.h:1404: warning: 'struct task_struct' declared inside parameter list
include/linux/blkdev.h:1404: warning: its scope is only this definition or declaration, which is probably not what you want
include/linux/blkdev.h:1408: warning: 'struct task_struct' declared inside parameter list
include/linux/blkdev.h:1413: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'blk_needs_flush_plug'
make[4]: *** [drivers/mmc/host/dw_mmc.o] Error 1

This is because dw_mmc.c includes linux/blkdev.h as the very first file,
and when CONFIG_BLOCK=n, blkdev.h omits all includes.

As it requires linux/sched.h even when CONFIG_BLOCK=n, move this out of
the #ifdef.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>