Fix the build failure when built with newer gcc:
../gtk/gtktextsegment.c: In function '_gtk_toggle_segment_new':
../gtk/gtktextsegment.c:433:6: error: array subscript 'GtkTextLineSegment {aka struct _GtkTextLineSegment}[0]' is partly outside array bounds of 'unsigned char[40]' [-Werror=array-bounds]
433 | seg->type = on ? >k_text_toggle_on_type : >k_text_toggle_off_type;
| ^~
Patch source: b4e110920a