fitz/fitz.i: fixed #2355.

Fix from Harald, adds Outline.__del__() function similar to other classes.

[The bug seems to have been provoked by swig-4.1.1, it does not seem to occur
with swig-4.0.2.]

fitz/fitz.i

@@ -9083,6 +9083,12 @@ struct Outline {
         def dest(self):
             '''outline destination details'''
             return linkDest(self, None)
+
+        def __del__(self):
+            if not isinstance(self, Outline):
+                return
+            if getattr(self, "thisown", False):
+                self.__swig_destroy__(self)
         %}
     }
 };