lavu/spherical: Make AVSphericalMapping pointer parameter const.

Reflects the actual code and silences a gcc warning:
ffprobe.c:1797:42: warning: passing argument 1 of 'av_spherical_tile_bounds' discards 'const' qualifier from pointer target type

libavutil/spherical.c

@@ -33,7 +33,7 @@ AVSphericalMapping *av_spherical_alloc(size_t *size)
     return spherical;
 }
 
-void av_spherical_tile_bounds(AVSphericalMapping *map,
+void av_spherical_tile_bounds(const AVSphericalMapping *map,
                               size_t width, size_t height,
                               size_t *left, size_t *top,
                               size_t *right, size_t *bottom)

libavutil/spherical.h

@@ -202,7 +202,7 @@ AVSphericalMapping *av_spherical_alloc(size_t *size);
  * @param right  Pixels from the right edge.
  * @param bottom Pixels from the bottom edge.
  */
-void av_spherical_tile_bounds(AVSphericalMapping *map,
+void av_spherical_tile_bounds(const AVSphericalMapping *map,
                               size_t width, size_t height,
                               size_t *left, size_t *top,
                               size_t *right, size_t *bottom);