clang 23 added -Wunused-but-set-global, which fires on the
type_name##_parent_class variable that _G_DEFINE_TYPE_EXTENDED_BEGIN_PRE
emits whenever a type never chains up to its parent:
error: variable 'my_thing_parent_class' set but not used
[-Werror,-Wunused-but-set-global]
Projects that promote the unused-but-set group to an error (pango, for
one) now fail to build with clang 23, and they cannot fix it on their
side because the variable comes from this macro.
Annotate it G_GNUC_UNUSED, the same treatment the adjacent
get_instance_private helper already gets.
Signed-off-by: Sunil Dora <sunilkumar.dora@windriver.com>
Fixes: #4038
1567250294