patch 9.0.1001: classes are not documented or implemented yet

Problem:    Classes are not documented or implemented yet.
Solution:   Make the first steps at documenting Vim9 objects, classes and
            interfaces.  Make initial choices for the syntax.  Add a skeleton
            implementation.  Add "public" and "this" in the command table.
diff --git a/src/proto.h b/src/proto.h
index 9d1e134..50802ce 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -215,6 +215,7 @@
 # ifdef FEAT_EVAL
 // include vim9.h here, the types defined there are used by function arguments.
 #  include "vim9.h"
+#  include "vim9class.pro"
 #  include "vim9cmds.pro"
 #  include "vim9compile.pro"
 #  include "vim9execute.pro"