client/widgets/Buttons.cpp: Fix const reference

Co-authored-by: Andrey Filipenkov <[email protected]>

client/widgets/Buttons.cpp

@@ -66,7 +66,7 @@ void CButton::setBorderColor(std::optional<ColorRGBA> newBorderColor)
 	borderColor = newBorderColor;
 }
 
-void CButton::addCallback(std::function<void()> const & callback)
+void CButton::addCallback(const std::function<void()> & callback)
 {
 	this->callback += callback;
 }