Fix MQTT discovery for sensor state_class (#2331)

esphome/components/sensor/sensor.cpp

@@ -6,7 +6,7 @@ namespace sensor {
 
 static const char *const TAG = "sensor";
 
-const char *state_class_to_string(StateClass state_class) {
+std::string state_class_to_string(StateClass state_class) {
   switch (state_class) {
     case STATE_CLASS_MEASUREMENT:
       return "measurement";