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/ex_cmds.h b/src/ex_cmds.h
index 5ec69be..4e5c0aa 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -401,6 +401,9 @@
 EX(CMD_confirm,		"confirm",	ex_wrongmodifier,
 			NEEDARG|EXTRA|NOTRLCOM|CMDWIN,
 			ADDR_NONE),
+EX(CMD_const,		"const",	ex_const,
+			EXTRA|NOTRLCOM|SBOXOK|CMDWIN,
+			ADDR_NONE),
 EX(CMD_copen,		"copen",	ex_copen,
 			RANGE|COUNT|TRLBAR,
 			ADDR_OTHER),