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

ndk/platforms/android-3/include/netinet/in6.h

@@ -103,7 +103,6 @@
 #define IPV6_LEAVE_GROUP IPV6_DROP_MEMBERSHIP
 
 #define IN6ADDR_ANY_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}}}
-#define IN6ADDR_LOOPBACK_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}}}
 
 #define ipv6mr_interface ipv6mr_ifindex