The kbuild robot reports that we're relying on an implicit inclusion to
get a definition of task_stack_page() in the Spectre-v4 mitigation code,
which is not always in place for some configurations:

  | arch/arm64/kernel/proton-pack.c:329:2: error: implicit declaration of function 'task_stack_page' [-Werror,-Wimplicit-function-declaration]
  |         task_pt_regs(task)->pstate |= val;
  |         ^
  | arch/arm64/include/asm/processor.h:268:36: note: expanded from macro 'task_pt_regs'
  |         ((struct pt_regs *)(THREAD_SIZE + task_stack_page(p)) - 1)
  |                                           ^
  | arch/arm64/kernel/proton-pack.c:329:2: note: did you mean 'task_spread_page'?

Add the missing include to fix the build error.

Fixes: a44acf477220 ("arm64: Move SSBD prctl() handler alongside other spectre mitigation code")
Reported-by: Anthony Steinhauser <asteinhauser@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/202009260013.Ul7AD29w%lkp@intel.comSigned-off-by: Will Deacon <will@kernel.org>