um: Fix warning in setup_signal_stack_si()

Fixes:
arch/x86/um/signal.c: In function ‘setup_signal_stack_si’:
include/asm-generic/uaccess.h:146:27: warning: initialization from incompatible pointer type [enabled by default]
  __typeof__(*(ptr)) __x = (x);    \
                           ^
arch/x86/um/signal.c:544:10: note: in expansion of macro ‘__put_user’
   err |= __put_user(ksig->ka.sa.sa_restorer,

Signed-off-by: Richard Weinberger <richard@nod.at>