x86: ia32_signal: use proper macro __USER32_DS

Impact: cleanup

Use __USER32_DS instead of __USER_DS in ia32_signal.c.
No impact, because __USER32_DS is defined __USER_DS.

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