Make the network headers compile standalone.

Bug: http://b/28067717
Change-Id: I0f69c6cba0eb0df2fe68f458ba5aa81fe8e6434c
diff --git a/libc/include/netinet/ip6.h b/libc/include/netinet/ip6.h
index aa816c2..aa89186 100644
--- a/libc/include/netinet/ip6.h
+++ b/libc/include/netinet/ip6.h
@@ -64,6 +64,10 @@
 #ifndef _NETINET_IP6_H_
 #define _NETINET_IP6_H_
 
+#include <sys/types.h>
+
+#include <linux/in6.h>
+
 /*
  * Definition for internet protocol version 6.
  * RFC 2460