patch 8.1.1539: not easy to define a variable and lock it
Problem: Not easy to define a variable and lock it.
Solution: Add ":const".
diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index f501300..5e800ad 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -28,6 +28,7 @@
void *call_func_retlist(char_u *func, int argc, typval_T *argv);
int eval_foldexpr(char_u *arg, int *cp);
void ex_let(exarg_T *eap);
+void ex_const(exarg_T *eap);
void list_hashtable_vars(hashtab_T *ht, char *prefix, int empty, int *first);
char_u *get_lval(char_u *name, typval_T *rettv, lval_T *lp, int unlet, int skip, int flags, int fne_flags);
void clear_lval(lval_T *lp);