Wine uses extended attributes to implement Windows ACL. This is needed when using Microsoft PlayReady (e.g. DRM-protected media via Silverlight). Unfortunately Wine's dependency on sys-apps/attr is automagic. It simply checks for the presence and usability of attr/xattr.h regardless. I doubt a Wine bug report about this will get addressed in a hurry. I'd be inclined to just add it as a mandatory dependency.
First, what version of wine are you using? Second, please explain where wine checks for xattr.h. When I grep wine-1.6 and wine-1.7.1 source code for xattr.h, I do not see any results.
Ah! My mistake. I spotted this while applying the "compholio" patches to make Pipelight (see bug #481596) work. I didn't realise that xattr.h had been introduced by the patches. I was planning to open another bug report to get these patches included but I'll resolve this issue first. Please close this as INVALID.