Only define sa_family_t once.

Change-Id: Icf700910c836d22c8a27ad026ec8af11697edd5a
diff --git a/libc/include/sys/un.h b/libc/include/sys/un.h
index 65ffbdcf..421b900 100644
--- a/libc/include/sys/un.h
+++ b/libc/include/sys/un.h
@@ -25,11 +25,11 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+
 #ifndef _SYS_UN_H_
 #define _SYS_UN_H_
 
-typedef unsigned short sa_family_t;
-
+#include <bits/sa_family_t.h>
 #include <linux/un.h>
 
-#endif /* _SYS_UN_H_ */
+#endif