softlockup: fix to allow compiling with !DETECT_HUNG_TASK

Fixes the following compile error:

 kernel/fork.c:1049: error: 'struct task_struct' has no member named 'last_switch_count'
 kernel/fork.c:1050: error: 'struct task_struct' has no member named 'last_switch_timestamp'

Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>