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/gui_w32.c b/src/gui_w32.c
index c2371f4..61b09e1 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -6720,7 +6720,7 @@
     char_u	*buttons,
     int		 dfltbutton,
     char_u	*textfield,
-    int		ex_cmd)
+    int		ex_cmd UNUSED)
 {
     WORD	*p, *pdlgtemplate, *pnumitems;
     DWORD	*dwp;
@@ -8517,7 +8517,7 @@
 
     BalloonEval *
 gui_mch_create_beval_area(
-    void	*target,	/* ignored, always use s_textArea */
+    void	*target UNUSED,	/* ignored, always use s_textArea */
     char_u	*mesg,
     void	(*mesgCB)(BalloonEval *, int),
     void	*clientData)