inet6: Include if_private.h in one more netstack file

ip6_input() and ip6_destroy() both directly reference ifnet members.
This file was missed in 3d0d5b21

Fixes:		3d0d5b21 ("IfAPI: Explicitly include <net/if_private.h>...")
Sponsored by:	Juniper Networks, Inc.

sys/netinet6/ip6_input.c

@@ -95,6 +95,7 @@ __FBSDID("$FreeBSD$");
 #include <net/if.h>
 #include <net/if_var.h>
 #include <net/if_types.h>
+#include <net/if_private.h>
 #include <net/if_dl.h>
 #include <net/route.h>
 #include <net/netisr.h>