patch 9.0.0792: MS-Windows: compiler complains about unused function

Problem:    MS-Windows: compiler complains about unused function.
Solution:   Add #ifdef. (John Marriott)
diff --git a/src/os_win32.c b/src/os_win32.c
index db43c9e..5c845c6 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -1231,7 +1231,7 @@
 }
 # endif
 
-
+#ifdef VIMDLL
 /*
  * Win32 console mouse scroll event handler.
  * Loosely based on the _OnMouseWheel() function in gui_w32.c
@@ -1324,6 +1324,7 @@
 
     return;
 }
+#endif
 
 /*
  * Decode a MOUSE_EVENT.  If it's a valid event, return MOUSE_LEFT,