Cannot forward-declare struct

On gcc 3.x I get `nuklear/nuklear.h:3634: warning: parameter has incomplete type` because of a forward-declaration for `struct nk_style_item` for which it cannot determine the size of. This struct also includes members of type `struct nk_style_item_data` and `struct nk_style_item_type` which also receive the same errors if I do not move all 3 definitions above this line.

Is there a good solution to this problem?

Start at nuklear/nuklear.h:3634 and inspect the declarations of struct nk_style_item, struct nk_style_item_data, and struct nk_style_item_type. Reproduce the warning with the reported GCC 3.x setup if available, then verify that the header compiles without incomplete-type warnings while preserving the existing API.

Written by the indexing model from the issue text.