[IPV6]: net/addrconf.h needs to include linux/in6.h earlier

Else the in6_addr layout is not known for struct
prefix_info.
Signed-off-by: NPatrick McHardy <kaber@trash.net>

include/net/addrconf.h

@@ -17,6 +17,8 @@
 
 #define IPV6_MAX_ADDRESSES		16
 
+#include <linux/in6.h>
+
 struct prefix_info {
 	__u8			type;
 	__u8			length;
@@ -43,7 +45,6 @@ struct prefix_info {
 
 #ifdef __KERNEL__
 
-#include <linux/in6.h>
 #include <linux/netdevice.h>
 #include <net/if_inet6.h>
 #include <net/ipv6.h>