x86: signal_64.c: arg for restore_i387_xstate() is void __user *

restore_i387_xstate() is declared as:

  int restore_i387_xstate(void __user *buf);

so, make the variable buf void __user *.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>