Fix build of the widgets doc snippets

Rename main() of the graphicsview.cpp snippet to ensure it
is not considered a real main() function and prevent future
duplicate symbols.

Amends 2e1ef96cf8.

Pick-to: 5.15
Task-number: QTBUG-81497
Change-Id: Ibbb29e17f263ff02d3910852b195b1513b953f75
Reviewed-by: Kai Koehne <kai.koehne@qt.io>

src/widgets/doc/snippets/graphicsview.cpp

@@ -56,7 +56,7 @@
 #include <QDrag>
 #include <QGraphicsSceneMouseEvent>
 
-void main()
+void graphicsview_snippet_main()
 {
 //! [0]
 QGraphicsScene scene;