Fix build with bluetooth enabled

We need to implement this pure virtual method.

This is just a placeholder and lacks a proper implementation.

core/backends/bluetooth/bluetoothdevicelink.cpp

@@ -89,3 +89,8 @@ void BluetoothDeviceLink::dataReceived()
         QMetaObject::invokeMethod(this, &BluetoothDeviceLink::dataReceived, Qt::QueuedConnection);
     }
 }
+
+QSslCertificate BluetoothDeviceLink::certificate() const
+{
+    return {}; // TODO Not sure what to do here. For LanDeviceLink we use the SSL connection's certificate, but we don't have that here
+}

core/backends/bluetooth/bluetoothdevicelink.h

@@ -10,6 +10,7 @@
 #include <QObject>
 #include <QString>
 #include <QBluetoothSocket>
+#include <QSslCertificate>
 
 #include "../devicelink.h"
 #include "../devicelinereader.h"
@@ -33,6 +34,7 @@ public:
     virtual void userRequestsUnpair() override;
 
     virtual bool linkShouldBeKeptAlive() override;
+    QSslCertificate certificate() const override;
 
 private Q_SLOTS:
     void dataReceived();