s390: add missing declarations

arch_dup_task_struct and the per cpu variable mt_cycles are globally
visible, but do not have any header file with a declaration.
Therefore add it so we have proper type checking in place.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>