Fix compile error in blk-exec.c for !CONFIG_DETECT_HUNG_TASK

Ensure that 'sysctl_hung_task_timeout_secs' is defined
even when CONFIG_DETECT_HUNG_TASK is not set.
This way we can safely reference it without need for
ifdefs in the code elsewhere.  eg. in block/blk-exec.c

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>