usermod/sensors_to_mqtt: Add explicit dep

This mod includes a header from the Adafruit Unified Sensor library
inherited by its target sensor libraries.  This isn't reliably
picked up by PlatformIO's dependency finder.  Add an explicit dep to
ensure build stability.

usermods/sensors_to_mqtt/library.json

@@ -4,6 +4,7 @@
   "dependencies": {
     "adafruit/Adafruit BMP280 Library":"2.6.8",
     "adafruit/Adafruit CCS811 Library":"1.1.3",
-    "adafruit/Adafruit Si7021 Library":"1.5.3"
+    "adafruit/Adafruit Si7021 Library":"1.5.3",
+    "adafruit/Adafruit Unified Sensor":"^1.1.15"
   }
 }