[posix] make base class destructor virtual (openthread#7220)

Author: rxwen

src/posix/platform/mainloop.hpp

@@ -65,6 +65,12 @@ class Source
      */
     virtual void Process(const otSysMainloopContext &aContext) = 0;
 
+    /**
+     * This method marks desturctor virtual method.
+     *
+     */
+    virtual ~Source() = default;
+
 private:
     Source *mNext = nullptr;
 };