Fix TLS RPT report parser by ignoring the 'mx-host' field

It can be either a scalar or a list, skip parsing it for now as it is unused.

tlsrpt.go

@@ -23,7 +23,10 @@ type tlsrptPolicy struct {
 		Type   string   `json:"policy-type"`
 		Policy []string `json:"policy-string"`
 		Domain string   `json:"policy-domain"`
-		Mx     string   `json:"mx-host"`
+		// TODO: apparently the following field can be a
+		// scalar or an array. Drop it for now as we're not
+		// using it.
+		//Mx []string `json:"mx-host"`
 	} `json:"policy"`
 	Summary struct {
 		NumSuccessful int `json:"total-successful-session-count"`