patch 9.0.1031: Vim9 class is not implemented yet

Problem:    Vim9 class is not implemented yet.
Solution:   Add very basic class support.
diff --git a/src/proto/vim9instr.pro b/src/proto/vim9instr.pro
index 87914f3..04d7682 100644
--- a/src/proto/vim9instr.pro
+++ b/src/proto/vim9instr.pro
@@ -3,6 +3,7 @@
 isn_T *generate_instr_drop(cctx_T *cctx, isntype_T isn_type, int drop);
 isn_T *generate_instr_type(cctx_T *cctx, isntype_T isn_type, type_T *type);
 isn_T *generate_instr_debug(cctx_T *cctx);
+int generate_CONSTRUCT(cctx_T *cctx, class_T *cl);
 int may_generate_2STRING(int offset, int tolerant, cctx_T *cctx);
 int generate_add_instr(cctx_T *cctx, vartype_T vartype, type_T *type1, type_T *type2, exprtype_T expr_type);
 vartype_T operator_type(type_T *type1, type_T *type2);