ndk: Fix ipv6 related headers

This fixes two minor issues with recent updates to the IPv6 related headers:

- add missing <linux/ipv6.h> kernel header
- avoid conflict in netinet/in6.h, the macro is already defined by <linux/in6.h>

Change-Id: Ibd0d44fd801affc6ca405e96982a73f9a630bdba
2 files changed