patch 8.2.1745: tiny version doesn't build

Problem:    Tiny version doesn't build.
Solution:   Add dummy ex_var() function.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index f584153..a7542f3 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -291,6 +291,7 @@
 # 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