Explicit destructors

src/TinyGsmClientMC60.h

@@ -61,6 +61,8 @@ public:
     init(&modem, mux);
   }
 
+  virtual ~GsmClient(){}
+
   bool init(TinyGsmMC60* modem, uint8_t mux = 1) {
     this->at = modem;
     this->mux = mux;
@@ -126,6 +128,8 @@ private:
 //     : GsmClient(modem, mux)
 //   {}
 //
+//   virtual ~GsmClientSecure(){}
+//
 // public:
 //   virtual int connect(const char *host, uint16_t port, int timeout_s) {
 //     stop();
@@ -145,6 +149,8 @@ public:
     memset(sockets, 0, sizeof(sockets));
   }
 
+  virtual ~TinyGsmMC60() {}
+
   /*
    * Basic functions
    */