Fix sys/un.h to be included on its own

See https://code.google.com/p/android/issues/detail?id=53711
https://android-review.googlesource.com/#/c/42763

Change-Id: Ibd8a445dfde752cddab61b3ae53ad077738701ea
diff --git a/ndk/platforms/android-3/include/sys/un.h b/ndk/platforms/android-3/include/sys/un.h
index 973861f..f89ead3 100644
--- a/ndk/platforms/android-3/include/sys/un.h
+++ b/ndk/platforms/android-3/include/sys/un.h
@@ -28,6 +28,9 @@
 #ifndef _SYS_UN_H_
 #define _SYS_UN_H_
 
+#include <sys/_types.h>
+typedef __sa_family_t sa_family_t;
+
 #include <linux/un.h>
 
 #endif /* _SYS_UN_H_ */