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>

arch/x86/um/signal.c

@@ -541,7 +541,8 @@ int setup_signal_stack_si(unsigned long stack_top, struct ksignal *ksig,
 	 */
 	/* x86-64 should always use SA_RESTORER. */
 	if (ksig->ka.sa.sa_flags & SA_RESTORER)
-		err |= __put_user(ksig->ka.sa.sa_restorer, &frame->pretcode);
+		err |= __put_user((void *)ksig->ka.sa.sa_restorer,
+				  &frame->pretcode);
 	else
 		/* could use a vstub here */
 		return err;