fix compiler warning in SHTSensor::setAccuracy()

SHTSensor::setAccuracy(SHTAccuracy newAccuracy) is simply returning
false in the base class, since most of the sensors don't support
it. as such, 'newAccuracy' is newer used.

Mark it as unused to get rid of the compiler warning in restrictive
compilation setups

Author: winkj

SHTSensor.h

@@ -157,7 +157,7 @@ class SHTSensorDriver
    * Set the sensor accuracy.
    * Returns false if the sensor does not support changing the accuracy
    */
-  virtual bool setAccuracy(SHTSensor::SHTAccuracy newAccuracy) {
+  virtual bool setAccuracy(__attribute__((unused)) SHTSensor::SHTAccuracy newAccuracy) {
     return false;
   }