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_getln.c b/src/ex_getln.c
index ef9967e..ef8e387 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -3754,7 +3754,7 @@
  * Returns the result in allocated memory.
  */
     char_u *
-vim_strsave_fnameescape(char_u *fname, int shell)
+vim_strsave_fnameescape(char_u *fname, int shell UNUSED)
 {
     char_u	*p;
 #ifdef BACKSLASH_IN_FILENAME