[DISC] Add HASS_UNIT_PPM to availableHASSUnits array (#2279)

HASS_UNIT_PPM is defined in config_mqttDiscovery.h but was missing from
the availableHASSUnits validation array in mqttDiscovery.cpp. This caused
unit_of_measurement to be silently dropped for CO2 sensors (like the
Govee H5140) during MQTT discovery, resulting in Home Assistant warnings
about invalid units.

main/mqttDiscovery.cpp

@@ -127,6 +127,7 @@ static const char* const availableHASSUnits[] = {
     HASS_UNIT_UGM3,
     HASS_UNIT_OHM,
     HASS_UNIT_PERCENT,
+    HASS_UNIT_PPM,
     HASS_UNIT_DEGREE,
     HASS_UNIT_CELSIUS,
     HASS_UNIT_FAHRENHEIT,