patch 9.0.1369: still some "else if" constructs for setting options

Problem:    Still some "else if" constructs for setting options.
Solution:   Add a few more functions for handling options. (Yegappan
            Lakshmanan, closes #12090)
diff --git a/src/proto/optionstr.pro b/src/proto/optionstr.pro
index e021ad3..3163ba7 100644
--- a/src/proto/optionstr.pro
+++ b/src/proto/optionstr.pro
@@ -20,6 +20,7 @@
 char *did_set_bufhidden(optset_T *args);
 char *did_set_buftype(optset_T *args);
 char *did_set_casemap(optset_T *args);
+char *did_set_chars_option(optset_T *args);
 char *did_set_cinoptions(optset_T *args);
 char *did_set_colorcolumn(optset_T *args);
 char *did_set_comments(optset_T *args);