Fix support for Qt4 plural forms format strings.
Pinpointed by gcc 13 warning:
warning: missing initializer for field ‘is_format3’ of ‘flag_context_ty’ [-Wmissing-field-initializers]
* gettext-tools/src/xg-arglist-context.c (null_context, passthrough_context,
passthrough_context_circular_list): Initialize also the fields is_format3,
pass_format3.
