QXpmHandler: clean up write_xpm_image: make w,h const

... and use one variable definition per line.

Change-Id: Ie8e6d6fb9e570cf715ab0b39c78d677f1e5a19a7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/gui/image/qxpmhandler.cpp

@@ -1118,7 +1118,9 @@ static bool write_xpm_image(const QImage &sourceImage, QIODevice *device, const
 
     QMap<QRgb, int> colorMap;
 
-    int w = image.width(), h = image.height(), ncolors = 0;
+    const int w = image.width();
+    const int h = image.height();
+    int ncolors = 0;
     int x, y;
 
     // build color table