Include sys/types.h for ssize_t on OS/2

This fixes the following error,

In file included from ../../../include/vlc_stream.h:27,
                 from ../../../include/vlc_demux.h:33,
                 from mkv.hpp:55,
                 from demux.hpp:28,
                 from virtual_segment.cpp:27:
../../../include/vlc_block.h:138: error: 'ssize_t' has not been declared
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

include/vlc_block.h

@@ -30,6 +30,8 @@
  *
  */
 
+#include <sys/types.h>  /* for ssize_t */
+
 /****************************************************************************
  * block:
  ****************************************************************************