declare wifi mode prior to connect

must have statement for current Arduino Framework, see also https://github.com/knolleary/pubsubclient/issues/138#issuecomment-326113915

examples/mqtt_esp8266/mqtt_esp8266.ino

@@ -42,6 +42,7 @@ void setup_wifi() {
   Serial.print("Connecting to ");
   Serial.println(ssid);
 
+  WiFi.mode(WIFI_STA);
   WiFi.begin(ssid, password);
 
   while (WiFi.status() != WL_CONNECTED) {