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/memline.c b/src/memline.c
index 08a99f7..c77d1df 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -2101,7 +2101,7 @@
     struct tm *
 vim_localtime(
     const time_t	*timep,		// timestamp for local representation
-    struct tm		*result)	// pointer to caller return buffer
+    struct tm		*result UNUSED)	// pointer to caller return buffer
 {
 #ifdef HAVE_LOCALTIME_R
 # ifdef HAVE_TZSET