[MINOR] http: rely on proxy->acl_requires to allocate hdr_idx

[MINOR] http: rely on proxy->acl_requires to allocate hdr_idx

Right now only HTTP proxies may use HTTP headers in ACLs, but
when this evolves, we'll need to be able to allocate the hdr_idx
on demand. The solution consists in allocating it only when it is
certain that at least one ACL requires HTTP parsing, regardless
of the mode the proxy is in. This is what is achieved by this
patch.