FROMLIST: arm64: use the correct function type in SYSCALL_DEFINE0

Although a syscall defined using SYSCALL_DEFINE0 doesn't accept
parameters, use the correct function type to avoid indirect call
type mismatches with Control-Flow Integrity checking.

Bug: 133186739
Change-Id: I45d1235f4b93c69451ed3610b676d0948cef5ee1
Link: https://lkml.org/lkml/2019/5/3/691


Signed-off-by: Sami Tolvanen <samitolvanen@google.com>