Do not use libdvdnav's read ahead cache on OS/2.

Using it causes libdvdnav to complain about 'demux error'.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit bdf3c466d2212343667f5cda1bce3938d17471cd)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/access/dvdnav.c

@@ -97,7 +97,11 @@ vlc_module_begin ()
 vlc_module_end ()
 
 /* Shall we use libdvdnav's read ahead cache? */
+#ifdef __OS2__
+#define DVD_READ_CACHE 0
+#else
 #define DVD_READ_CACHE 1
+#endif
 
 /*****************************************************************************
  * Local prototypes