[PHY] Add a virtual destructor (#1557)

* Update PhysicalLayer.h

Add a virtual destructor, so can use new + delete on inherited objects

* Update PhysicalLayer.h

deleted empty lines

src/protocols/PhysicalLayer/PhysicalLayer.h

@@ -229,6 +229,11 @@ class PhysicalLayer {
     */
     PhysicalLayer();
 
+    /*!
+      \brief Default destructor.
+    */
+    virtual ~PhysicalLayer() = default;
+
     // basic methods
 
     #if defined(RADIOLIB_BUILD_ARDUINO)