netfilter: install missing headers

iptables imports headers from (the unifdefed headers of a)
kernel tree, but some headers happened to not be installed.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/include/linux/netfilter_ipv6/Kbuild b/include/linux/netfilter_ipv6/Kbuild
index 8887a5f..aca4bd1 100644
--- a/include/linux/netfilter_ipv6/Kbuild
+++ b/include/linux/netfilter_ipv6/Kbuild
@@ -11,6 +11,7 @@
 header-y += ip6t_limit.h
 header-y += ip6t_mac.h
 header-y += ip6t_mark.h
+header-y += ip6t_mh.h
 header-y += ip6t_multiport.h
 header-y += ip6t_opts.h
 header-y += ip6t_owner.h