QColorSpace: tidies

It doesn't actually need Q_DECLARE_PRIVATE (it never uses Q_D),
it just needs to give friendship to the private class.

Change-Id: I32ac128118a0c319a374e510a18ccf0edf323da7
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/painting/qcolorspace.h

@@ -124,7 +124,7 @@ public:
     operator QVariant() const;
 
 private:
-    Q_DECLARE_PRIVATE(QColorSpace)
+    friend class QColorSpacePrivate;
     QColorSpacePrivate *d_ptr = nullptr;
 
 #ifndef QT_NO_DEBUG_STREAM