picture_pool: fix prototype, fix warnings

include/vlc_picture_pool.h

@@ -131,7 +131,7 @@ VLC_USED;
 /**
  * It returns the size of the given pool.
  */
-VLC_API int picture_pool_GetSize(picture_pool_t *);
+VLC_API unsigned picture_pool_GetSize(const picture_pool_t *);
 
 
 #endif /* VLC_PICTURE_POOL_H */

src/misc/picture_pool.c

@@ -326,7 +326,7 @@ void picture_pool_NonEmpty(picture_pool_t *pool)
     vlc_mutex_unlock(&pool->lock);
 }
 
-int picture_pool_GetSize(picture_pool_t *pool)
+unsigned picture_pool_GetSize(const picture_pool_t *pool)
 {
     return pool->picture_count;
 }