syscalls/x86: Use the correct function type for sys_ni_syscall

Use the correct function type for sys_ni_syscall() in system
call tables to fix indirect call mismatches with Control-Flow
Integrity (CFI) checking.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: H . Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191008224049.115427-5-samitolvanen@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>