gobject: fix G_DEFINE_TYPE_EXTENDED docs so code snippet actually compiles

gobject: fix G_DEFINE_TYPE_EXTENDED docs so code snippet actually compiles

Flags being used in the G_DEFINE_TYPE_EXTENDED sample was "0", so it
should expand to 0 as well, otherwise the compiler would bark with:
maman-bar.c: In function ‘maman_bar_get_type’:
maman-bar.c:36:53: error: ‘flags’ undeclared (first use in this function)
maman-bar.c:36:53: note: each undeclared identifier is reported only once for each function it appears in

https://bugzilla.gnome.org/show_bug.cgi?id=697250