Fix linker failure when using BluetoothSerial on latest release with Arduino as Component. (espressif#8784)

Author: blackhack

libraries/BluetoothSerial/src/BTAdvertisedDevice.h

@@ -15,17 +15,17 @@ class BTAdvertisedDevice {
 public:
     virtual ~BTAdvertisedDevice() = default;
 
-    virtual BTAddress   getAddress();
-    virtual uint32_t    getCOD() const;
-    virtual std::string getName() const;
-    virtual int8_t      getRSSI() const;
+    virtual BTAddress   getAddress() = 0;
+    virtual uint32_t    getCOD() const = 0;
+    virtual std::string getName() const = 0;
+    virtual int8_t      getRSSI() const = 0;
 
 
-    virtual bool        haveCOD() const;
-    virtual bool        haveName() const;
-    virtual bool        haveRSSI() const;
+    virtual bool        haveCOD() const = 0;
+    virtual bool        haveName() const = 0;
+    virtual bool        haveRSSI() const = 0;
 
-    virtual std::string toString();
+    virtual std::string toString() = 0;
 };
 
 class BTAdvertisedDeviceSet : public virtual BTAdvertisedDevice {

libraries/BluetoothSerial/src/BTScan.h

@@ -22,9 +22,9 @@ class BTScanResults {
 public:
     virtual ~BTScanResults() = default;
 
-    virtual void                dump(Print *print = nullptr);
-    virtual int                 getCount();
-    virtual BTAdvertisedDevice* getDevice(int i);
+    virtual void                dump(Print *print = nullptr) = 0;
+    virtual int                 getCount() = 0;
+    virtual BTAdvertisedDevice* getDevice(int i) = 0;
 };
 
 class BTScanResultsSet : public BTScanResults {