Fix sensor usermod globals

These can be static locals instead; allows these usermods to build and
link together.

usermods/Si7021_MQTT_HA/Si7021_MQTT_HA.cpp

@@ -9,7 +9,7 @@
 #error "This user mod requires MQTT to be enabled."
 #endif
 
-Adafruit_Si7021 si7021;
+static Adafruit_Si7021 si7021;
 
 class Si7021_MQTT_HA : public Usermod
 {

usermods/Si7021_MQTT_HA/library.json

@@ -1,6 +1,7 @@
 {
   "name:": "Si7021_MQTT_HA",
   "dependencies": {
-    "finitespace/BME280":"3.0.0"
+    "finitespace/BME280":"3.0.0",
+    "adafruit/Adafruit Si7021 Library" : "1.5.3"
   }
 }

usermods/sensors_to_mqtt/sensors_to_mqtt.cpp

@@ -9,9 +9,9 @@
 #error "This user mod requires MQTT to be enabled."
 #endif
 
-Adafruit_BMP280 bmp;
-Adafruit_Si7021 si7021;
-Adafruit_CCS811 ccs811;
+static Adafruit_BMP280 bmp;
+static Adafruit_Si7021 si7021;
+static Adafruit_CCS811 ccs811;
 
 class UserMod_SensorsToMQTT : public Usermod
 {