Fix compiler warning

Author: dkulp

src/util/GPIOUtils.h

@@ -158,7 +158,7 @@ class GPIODCapabilities : public PinCapabilitiesFluent<GPIODCapabilities> {
     virtual void releaseGPIOD() const override;
 
     virtual int requestEventFile(bool risingEdge, bool fallingEdge) const override;
-    virtual int readEventFromFile() const;
+    virtual int readEventFromFile() const override;
 
     std::string gpioName;
 #ifdef HASGPIOD