lib/Kconfig.debug: fix help message for DEFAULT_HUNG_TASK_TIMEOUT

Added missing _secs in the help message of config DEFAULT_HUNG_TASK_TIMEOUT.

Signed-off-by: Jiaju Zhang <jjzhang@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>