xsk: make struct xsk_cb_desc available outside CONFIG_XDP_SOCKETS

The ice driver fails to build when CONFIG_XDP_SOCKETS is disabled.

drivers/net/ethernet/intel/ice/ice_base.c:533:21: error:
variable has incomplete type 'struct xsk_cb_desc'
        struct xsk_cb_desc desc = {};
                           ^
include/net/xsk_buff_pool.h:15:8: note:
forward declaration of 'struct xsk_cb_desc'
struct xsk_cb_desc;
       ^

Fixes: d68d707d

 ("ice: Support XDP hints in AF_XDP ZC mode")
Closes: https://lore.kernel.org/netdev/8b76dad3-8847-475b-aa17-613c9c978f7a@infradead.org/
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Larysa Zaremba <larysa.zaremba@intel.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20231219110205.1289506-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>