powerpc: Add missing prototype for arch_dup_task_struct()

In commit 55ccf3fe ("fork: move the real prepare_to_copy() users to
arch_dup_task_struct()") a new arch_dup_task_struct() was added
without a prototype declared in thread_info.h header. Fix the
following warning (treated as error in W=1):

  arch/powerpc/kernel/process.c:1609:5: error: no previous prototype for ‘arch_dup_task_struct’

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>