os2: include vlc_input.h

This fixes the following compilation error.

-----
  CC       os2/specific.lo
os2/specific.c: In function 'IPCHelperThread':
os2/specific.c:107:34: error: 'VLC_INPUT_OPTION_TRUSTED' undeclared (first use in this function)
                                  VLC_INPUT_OPTION_TRUSTED,
                                  ^
os2/specific.c:107:34: note: each undeclared identifier is reported only once for each function it appears in
-----
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/os2/specific.c

@@ -25,6 +25,7 @@
 #include <vlc_common.h>
 #include "../libvlc.h"
 #include <vlc_playlist.h>
+#include <vlc_input.h>
 #include <vlc_interface.h>
 #include <vlc_url.h>