sys/extaddr.h: don't declare struct iovec

The code never uses it so there's no need to forward declare it.

Sponsored by:	Innovate UK

sys/sys/extattr.h

@@ -71,8 +71,6 @@ int	extattr_check_cred(struct vnode *vp, int attrnamespace,
 #else
 #include <sys/cdefs.h>
 
-struct iovec;
-
 __BEGIN_DECLS
 int	extattrctl(const char *_path, int _cmd, const char *_filename,
 	    int _attrnamespace, const char *_attrname);