amqp: fix typo in ifdef name

So with this patch, the plugin builds and works on centos5 (w/ librabbitmq-c 0.1) as well as on ubuntu trusty (upcoming 14.04, which has librabbitmq-c 0.4.1 at the moment).

@tokkee, I'm not sure why `HAVE_AMQP_TCP_SOCKET_H` and not `HAVE_AMQP_TCP_SOCKET` as defined in https://github.com/tokkee/collectd/blob/sh/collectd-5.3/configure.in#L3544 though. Maybe my patch is wrong actually...