Fix Windows.h not found when building on Linux with MinGW

src/win/win_media_menu.c

@@ -2,7 +2,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <Windows.h>
+#include <windows.h>
 #include <86box/86box.h>
 #include <86box/cdrom.h>
 #include <86box/config.h>
@@ -526,4 +526,4 @@ HMENU
 media_menu_get_mo(int id)
 {
     return menus[MO_FIRST + id];
-}
+}