kernel/hung_task.c - fix sparse warnings

kernel/hung_task.c - fix sparse warnings

sparse complains:

    CHECK   kernel/hung_task.c
  kernel/hung_task.c:28:19: warning: symbol 'sysctl_hung_task_check_count' was not declared. Should it be static?
  kernel/hung_task.c:42:29: warning: symbol 'sysctl_hung_task_timeout_secs' was not declared. Should it be static?
  kernel/hung_task.c:47:29: warning: symbol 'sysctl_hung_task_check_interval_secs' was not declared. Should it be static?
  kernel/hung_task.c:49:19: warning: symbol 'sysctl_hung_task_warnings' was not declared. Should it be static?
  kernel/hung_task.c:61:28: warning: symbol 'sysctl_hung_task_panic' was not declared. Should it be static?
  kernel/hung_task.c:219:5: warning: symbol 'proc_dohung_task_timeout_secs' was not declared. Should it be static?

Add the appropriate header file to provide declarations.

Link: http://lkml.kernel.org/r/467.1548649525@turing-police.cc.vt.edu
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>