Fix build of the widgets doc snippets on Windows

- Fix warning about missing return from main()
- Turn it into a library to prevent link errors from missing main()
  (used by winmain()).

Amends a426326e99.

Pick-to: 5.15
Task-number: QTBUG-81497
Change-Id: Idbcb5c26538ee474bc80387d0a1dae3af7b53744
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/widgets/doc/snippets/graphicsview.cpp

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

src/widgets/doc/snippets/snippets.pro

@@ -1,7 +1,7 @@
 requires(qtHaveModule(widgets))
 requires(qtHaveModule(printsupport))
 
-TEMPLATE = app
+TEMPLATE = lib
 
 TARGET = widgets_snippets