netfilter: conntrack: remove CONFIG_NF_CONNTRACK check from nf_conntrack_acct.h.

There is a superfluous `#if IS_ENABLED(CONFIG_NF_CONNTRACK)` check
wrapping some function declarations.  Remove it.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>