um/x86: Fix build after x86 syscall changes

I didn't realize that um didn't include x86's asm/syscall.h.
Re-add a missing typedef.

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 034042cc ("x86/entry/syscalls: Move syscall table declarations into asm/syscalls.h")
Link: http://lkml.kernel.org/r/8d15b9a88f4fd49e3342757e0a34624ee5ce9220.1444696194.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>