OS/2: Include sys/types.h in vlc_configuration.h

This fixes the following error.

-----
make.exe[1]: Entering directory `F:/lang/work/vlc/vlc.git/modules/demux/mkv'
  CXX    libmkv_plugin_la-virtual_segment.lo
In file included from ../../../include/vlc_common.h:938,
                 from mkv.hpp:45,
                 from demux.hpp:28,
                 from virtual_segment.cpp:27:
../../../include/vlc_configuration.h:104: error: 'ssize_t' does not name a type
../../../include/vlc_configuration.h:106: error: 'ssize_t' does not name a type
In file included from mkv.hpp:53,
                 from demux.hpp:28,
                 from virtual_segment.cpp:27:
../../../include/vlc_charset.h:153: warning: 'warn_unused_result' attribute ignored
make.exe[1]: *** [libmkv_plugin_la-virtual_segment.lo] Error 1
make.exe[1]: Leaving directory `F:/lang/work/vlc/vlc.git/modules/demux/mkv'
make.exe: *** [all] Error 2
-----
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

include/vlc_configuration.h

@@ -32,6 +32,8 @@
  * It includes functions allowing to declare, get or set configuration options.
  */
 
+#include <sys/types.h>  /* for ssize_t */
+
 # ifdef __cplusplus
 extern "C" {
 # endif