patch 8.2.0587: compiler warning for unused variable

Problem:    Compiler warning for unused variable.
Solution:   Add UNUSED.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index cc7139d..c70a433 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -4783,7 +4783,7 @@
  * to "cmd_start" or has a white space character before it.
  */
     int
-ends_excmd2(char_u *cmd_start, char_u *cmd)
+ends_excmd2(char_u *cmd_start UNUSED, char_u *cmd)
 {
     int c = *cmd;