add virtual destructor

base classes without virtual destructors risk undefined behavior if their children are deleted through a pointer to base. see https://isocpp.org/wiki/faq/virtual-functions#virtual-dtors

Adafruit_Sensor.h

@@ -140,6 +140,7 @@ class Adafruit_Sensor {
  public:
   // Constructor(s)
   void constructor();
+  virtual ~Adafruit_Sensor() {}
 
   // These must be defined by the subclass
   virtual void enableAutoRange(bool enabled) {};