os2: missing #include

```--
  CC       os2/specific.lo
os2/specific.c: In function 'IPCHelperThread':
os2/specific.c:57:5: error: implicit declaration of function 'pl_Get' [-Werror=implicit-function-declaration]
os2/specific.c:57:30: warning: initialization makes pointer from integer without a cast [enabled by default]
cc1: some warnings being treated as errors
```

--
Signed-off-by: Francois Cartegnie <fcvlcdev@free.fr>

src/os2/specific.c

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