vlc_fs: include io.h on OS/2

This fixes many 'implicit declaration of function' errors due to read()
, close() and so on at various places. For example,
src/input/vlmshell.c.

include/vlc_fs.h

@@ -45,6 +45,9 @@ struct iovec;
 # define lseek lseek64
 #endif
 
+#ifdef __OS2__
+# include <io.h>
+#endif
 
 /**
  * \defgroup os Operating system