Fixed building with --debug=YES after b28e50b1.

Fixed building with --debug=YES after b28e50b1.

In file included from src/njs_main.h:37,
                 from src/njs_diyfp.c:12:
src/njs_atom.h: In function ‘njs_atom_to_value’:
src/njs_atom.h:54:31: error: invalid use of incomplete typedef
		‘njs_flathsh_descr_t’ {aka ‘struct njs_flathsh_descr_s’}
   54 |         njs_assert(atom_id < h->elts_count);
      |                               ^~
src/njs_assert.h:14:15: note: in definition of macro ‘njs_assert’