lib: fix signedness of SPU track ID in libvlc_video_set_spu()

Luckily, signed and unsigned int are compatible types.
(cherry picked from commit 5987f34d99b14b127e51d5bc68fc97e0bf71e7b3)

include/vlc/libvlc_media_player.h

@@ -1025,10 +1025,10 @@ LIBVLC_API libvlc_track_description_t *
  * Set new video subtitle.
  *
  * \param p_mi the media player
- * \param i_spu new video subtitle to select
+ * \param i_spu video subtitle track to select (i_id from track description)
  * \return 0 on success, -1 if out of range
  */
-LIBVLC_API int libvlc_video_set_spu( libvlc_media_player_t *p_mi, unsigned i_spu );
+LIBVLC_API int libvlc_video_set_spu( libvlc_media_player_t *p_mi, int i_spu );
 
 /**
  * Set new video subtitle file.

lib/video.c

@@ -308,7 +308,7 @@ libvlc_track_description_t *
     return libvlc_get_track_description( p_mi, "spu-es" );
 }
 
-int libvlc_video_set_spu( libvlc_media_player_t *p_mi, unsigned i_spu )
+int libvlc_video_set_spu( libvlc_media_player_t *p_mi, int i_spu )
 {
     input_thread_t *p_input_thread = libvlc_get_input_thread( p_mi );
     vlc_value_t list;