doc: Add back gdbus-object-manager-example

This requires Meson 0.52.0 for the return value of gnome.gtkdoc().

gio/tests/meson.build

@@ -431,6 +431,11 @@ test_extra_programs += {
   },
 }
 
+gdbus_example_objectmanager_sources = files(
+  'gdbus-example-objectmanager-client.c',
+  'gdbus-example-objectmanager-server.c',
+)
+
 if cc.get_id() != 'msvc' and cc.get_id() != 'clang-cl'
   test_extra_programs += {
     # These three are manual-run tests because they need a session bus but don't bring one up themselves