netfilter: conntrack: wrap two inline functions in config checks.

nf_conntrack_synproxy.h contains three inline functions.  The contents
of two of them are wrapped in CONFIG_NETFILTER_SYNPROXY checks and just
return NULL if it is not enabled.  The third does nothing if they return
NULL, so wrap its contents as well.

nf_ct_timeout_data is only called if CONFIG_NETFILTER_TIMEOUT is
enabled.  Wrap its contents in a CONFIG_NETFILTER_TIMEOUT check like the
other inline functions in nf_conntrack_timeout.h.

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