gallium: Remove pb_buffer.h include from pb_slab.h

pb_slab.h don't make use of anything defined in pb_buffer.h.
But removing it breaks Iris build because pb_buffer.h then includes
util/format/u_formats.h that Iris was using but not including.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33558>
2 files changed