netfilter: install missing ebtables headers for userspace

The ebt_ip6.h and ebt_nflog.h headers are not not known to Kbuild and
therefore not installed by make headers_install.  Fix that up.
Signed-off-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter_bridge/Kbuild

@@ -3,11 +3,13 @@ header-y += ebt_among.h
 header-y += ebt_arp.h
 header-y += ebt_arpreply.h
 header-y += ebt_ip.h
+header-y += ebt_ip6.h
 header-y += ebt_limit.h
 header-y += ebt_log.h
 header-y += ebt_mark_m.h
 header-y += ebt_mark_t.h
 header-y += ebt_nat.h
+header-y += ebt_nflog.h
 header-y += ebt_pkttype.h
 header-y += ebt_redirect.h
 header-y += ebt_stp.h