Port xattr-at.h to AIX 7.1

* lib/xattr-at.h: Include <errno.h> before testing whether
ENOATTR is defined.

lib/xattr-at.h

@@ -26,6 +26,7 @@
 # include <attr/xattr.h>
 #endif
 
+#include <errno.h>
 #ifndef ENOATTR
 # define ENOATTR ENODATA        /* No such attribute */
 #endif