Fixed a typo in libass.

(cherry picked from commit 01fdef51)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/libass.c

@@ -703,7 +703,7 @@ static ass_handle_t *AssHandleYield( decoder_t *p_dec )
     char *psz_font_dir = NULL;
 
 #if defined(WIN32)
-    const UINT uPath = GetSystemWindowsDirectory( NULL, 0 );
+    const UINT uPath = GetSystemWindowsDirectoryW( NULL, 0 );
     if( uPath > 0 )
     {
         wchar_t *psw_path = calloc( uPath + 1, sizeof(wchar_t) );