netfilter: ip6_tables: ip6t_ext_hdr is now static inline

We may hit this in xt_LOG:

net/built-in.o:xt_LOG.c:function dump_ipv6_packet:
	error: undefined reference to 'ip6t_ext_hdr'

happens with these config options:

CONFIG_NETFILTER_XT_TARGET_LOG=y
CONFIG_IP6_NF_IPTABLES=m

ip6t_ext_hdr is fairly small and it is called in the packet path.
Make it static inline.

Reported-by: Simon Kirby <sim@netnation.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>