Make GpuDevice::update non-pure virtual

Because Nvidia devices don't need the update.

plugins/global/gpu/GpuDevice.cpp

@@ -49,7 +49,10 @@ void GpuDevice::initialize()
     m_temperatureProperty->setName(i18nc("@title", "Temperature"));
     m_temperatureProperty->setPrefix(name());
     m_temperatureProperty->setUnit(KSysGuard::UnitCelsius);
+}
 
+void GpuDevice::update()
+{
 }
 
 void GpuDevice::makeSensors()

plugins/global/gpu/GpuDevice.h

@@ -19,7 +19,7 @@ public:
     ~GpuDevice() override = default;
 
     virtual void initialize();
-    virtual void update() = 0;
+    virtual void update();
 
 protected:
     virtual void makeSensors();