FROMLIST: arm64: fix syscall_fn_t type

Syscall wrappers in <asm/syscall_wrapper.h> use const struct pt_regs *
as the argument type. Use const in syscall_fn_t as well to fix indirect
call type mismatches with Control-Flow Integrity checking.

Bug: 133186739
Change-Id: I23d26aef5107985631a9aac500f2ceefc0b25419
Link: https://lkml.org/lkml/2019/5/3/689


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