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/os_mswin.c b/src/os_mswin.c
index 00b8caa..5fb4809 100644
--- a/src/os_mswin.c
+++ b/src/os_mswin.c
@@ -272,7 +272,7 @@
     void
 mch_settitle(
     char_u *title,
-    char_u *icon)
+    char_u *icon UNUSED)
 {
 # ifdef FEAT_GUI_MSWIN
 #  ifdef VIMDLL
@@ -1164,7 +1164,7 @@
 PrintHookProc(
 	HWND hDlg,	// handle to dialog box
 	UINT uiMsg,	// message identifier
-	WPARAM wParam,	// message parameter
+	WPARAM wParam UNUSED,	// message parameter
 	LPARAM lParam	// message parameter
 	)
 {