Fix BYTE_ORDER check in <netinet/ip6.h>.

Neither BYTE_ORDER nor BIG_ENDIAN were being defined anywhere, so they
were defaulting to zero, leading to BIG_ENDIAN values being used.

Bug: http://b/31496165
Change-Id: Ic6089895f39b86e7762bdb5b7d5b01feb0d2fd17
diff --git a/libc/include/netinet/ip6.h b/libc/include/netinet/ip6.h
index 639637e..fcfd5d6 100644
--- a/libc/include/netinet/ip6.h
+++ b/libc/include/netinet/ip6.h
@@ -66,6 +66,7 @@
 
 #include <sys/cdefs.h>
 #include <sys/types.h>
+#include <endian.h>
 
 #include <linux/in6.h>