fix "Breaks with gedit 3.36.1"

thanks to uwekel giving #35 (comment)

Author: Ildar Mulyukov

Diff for: sourcecodebrowser/plugin.py

@@ -413,7 +413,7 @@ def _load_active_document_symbols(self):
 
         document = self.window.get_active_document()
         if document:
-            location = document.get_location()
+            location = document.get_file().get_location()
             if location:
                 uri = location.get_uri()
                 self._log.debug("Loading %s...", uri)