qt: fix unused function warning in scicon.cpp

Enclose MakeSingleColorImage in an anonymous namespace to avoid a
unused function warning on Windows and MacOSX.

Github-Pull: #6143

src/qt/scicon.cpp

@@ -11,7 +11,9 @@
 #include <QPalette>
 #include <QPixmap>
 
-static void MakeSingleColorImage(QImage& img, const QColor& colorbase)
+namespace {
+
+void MakeSingleColorImage(QImage& img, const QColor& colorbase)
 {
     img = img.convertToFormat(QImage::Format_ARGB32);
     for (int x = img.width(); x--; )
@@ -24,6 +26,8 @@ static void MakeSingleColorImage(QImage& img, const QColor& colorbase)
     }
 }
 
+}
+
 QImage SingleColorImage(const QString& filename, const QColor& colorbase)
 {
     QImage img(filename);