dvdnav: fix compilation on OS/2

```--
access/dvdnav.c: In function 'ProbeDVD':
access/dvdnav.c:1469:6: error: implicit declaration of function 'read' [-Werror=implicit-function-declaration]
      || read( fd, iso_dsc, sizeof (iso_dsc) ) < (int)sizeof (iso_dsc)
      ^
access/dvdnav.c:1481:5: error: implicit declaration of function 'close' [-Werror=implicit-function-declaration]
     close( fd );
     ^
```

--
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

modules/access/dvdnav.c

@@ -41,6 +41,7 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <unistd.h>     /* close() */
 
 #include <vlc_common.h>
 #include <vlc_plugin.h>