Added WIN_IsWindows8OrGreater() for internal use

src/core/windows/SDL_windows.h

@@ -69,6 +69,9 @@ extern BOOL WIN_IsWindowsVistaOrGreater(void);
 /* Returns SDL_TRUE if we're running on Windows 7 and newer */
 extern BOOL WIN_IsWindows7OrGreater(void);
 
+/* Returns SDL_TRUE if we're running on Windows 8 and newer */
+extern BOOL WIN_IsWindows8OrGreater(void);
+
 /* You need to SDL_free() the result of this call. */
 extern char *WIN_LookupAudioDeviceName(const WCHAR *name, const GUID *guid);