src/dvd_read.c: fix a compilation warning on OS/2

```--
src/dvd_reader.c: In function `DVDOpen':
src/dvd_reader.c:561: warning: implicit declaration of function `isalpha'
```

--
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/dvd_reader.c

@@ -32,6 +32,7 @@
 #include <unistd.h>         /* chdir, getcwd */
 #include <limits.h>         /* PATH_MAX */
 #include <dirent.h>         /* opendir, readdir */
+#include <ctype.h>          /* isalpha */
 
 /* misc win32 helpers */
 #ifdef WIN32