patch 8.1.2064: MS-Windows: compiler warnings for unused arguments

Problem:    MS-Windows: compiler warnings for unused arguments.
Solution:   Add UNUSED. (Yegappan Lakshmanan, closes #4963)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index d67bfac..a4a8756 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -7335,7 +7335,7 @@
 
 #if (defined(FEAT_SESSION) || defined(FEAT_EVAL)) || defined(PROTO)
     int
-vim_mkdir_emsg(char_u *name, int prot)
+vim_mkdir_emsg(char_u *name, int prot UNUSED)
 {
     if (vim_mkdir(name, prot) != 0)
     {