s390/thread_info.h: fix task_struct declaration warning

Add missing forward declaration for task_struct.
The warning appears when the -Werror C compiler flag is being used.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>