Explicit destructors

src/TinyGsmClientM590.h

@@ -57,6 +57,8 @@ public:
     init(&modem, mux);
   }
 
+  virtual ~GsmClient(){}
+
   bool init(TinyGsmM590* modem, uint8_t mux = 1) {
     this->at = modem;
     this->mux = mux;
@@ -119,6 +121,8 @@ public:
     memset(sockets, 0, sizeof(sockets));
   }
 
+  virtual ~TinyGsmM590() {}
+
   /*
    * Basic functions
    */