include/linux/stacktrace.h: declare struct task_struct

include/linux/stacktrace.h:13: warning:
 'struct task_struct' declared inside parameter list

(This might be a hard error on sparc64, which uses this header and has
-Werror)

Reported-by: "Randy.Dunlap" <rdunlap@xenotime.net>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>