Fix compilation of rlm_eap

Mark rlm_eap_tnc connection_string const to cater to FR_CONF_OFFSET
invocation. This fixes the build of rlm_eap module.

The reported error was this:
src/modules/rlm_eap/types/rlm_eap_tnc/rlm_eap_tnc.c:67:2: error: initializer element is not constant
  { "connection_string", FR_CONF_OFFSET(PW_TYPE_STRING, rlm_eap_tnc_t, connection_string), "NAS Port: %{NAS-Port} NAS IP: %{NAS-IP-Address} NAS_PORT_TYPE: %{NAS-Port-Type}" },
  ^
src/modules/rlm_eap/types/rlm_eap_tnc/rlm_eap_tnc.c:67:2: error: (near initialization for ‘module_config[0].offset’)