arm64: Include linux/ptrace.h in asm/compat.h

Commit 9b064fc3

 (new helper:
compat_user_stack_pointer()) introduces a call to current_pt_regs()
which is defined in linux/ptrace.h, not currently included asm/compat.h.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>