NRF52Board.h: Mark getMCUTemperature() as virtual

The function in the derived class is virtual per definition. Mark it
to make this clearer to the reader.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

src/helpers/NRF52Board.h

@@ -12,7 +12,7 @@ protected:
 public:
   virtual void begin();
   virtual uint8_t getStartupReason() const override { return startup_reason; }
-  float getMCUTemperature() override;
+  virtual float getMCUTemperature() override;
   virtual void reboot() override { NVIC_SystemReset(); }
 };