patch 8.2.1745: tiny version doesn't build

Problem:    Tiny version doesn't build.
Solution:   Add dummy ex_var() function.

src/ex_docmd.c

@@ -291,6 +291,7 @@ static void	ex_tag_cmd(exarg_T *eap, char_u *name);
 # define ex_function		ex_ni
 # define ex_if			ex_ni
 # define ex_let			ex_ni
+# define ex_var			ex_ni
 # define ex_lockvar		ex_ni
 # define ex_oldfiles		ex_ni
 # define ex_options		ex_ni