Our kernel contains multiple of such definitions that are generally a sign for dead code. Most of them are related to some DRM glue so I wouldn't touch them. But you might be interested by the others.
$ ctfconv -d obj/bsd.gdb |grep "(0 bytes)" [282] STRUCT lock_list_entry (0 bytes) [373] STRUCT dma_buf_attachment (0 bytes) [478] STRUCT drm_agp_mem (0 bytes) [482] STRUCT address_space (0 bytes) [484] STRUCT drm_local_map (0 bytes) [528] STRUCT device_node (0 bytes) [542] STRUCT device_attribute (0 bytes) [585] STRUCT dentry (0 bytes) [898] STRUCT workqueue_struct (0 bytes) [961] STRUCT rcu_head (0 bytes) [1053] STRUCT radeon_mn (0 bytes) [1184] STRUCT seq_file (0 bytes) [1257] STRUCT drm_info_list (0 bytes) [1266] STRUCT kfd_dev (0 bytes) [2960] STRUCT pwm_device (0 bytes) [3111] STRUCT pid (0 bytes) [3115] STRUCT i915_mm_struct (0 bytes) [3117] STRUCT i915_mmu_object (0 bytes) [3197] STRUCT io_mapping (0 bytes) [3226] STRUCT intel_overlay_error_state (0 bytes) [3263] STRUCT i915_audio_component (0 bytes) [5986] STRUCT sr_concat (0 bytes) [6479] UNION (anon) (0 bytes) [7671] STRUCT ft_softc (0 bytes) [13181] STRUCT display_timing (0 bytes) [13190] STRUCT gpio_desc (0 bytes) [14155] STRUCT scsibus (0 bytes) [16264] STRUCT ahc_tmode_lstate (0 bytes) [17282] STRUCT bge_tx_desc (0 bytes) [17284] STRUCT bge_cmd_desc (0 bytes) [17943] STRUCT gdt_ucmd (0 bytes) [21151] STRUCT arc4random_ctx (0 bytes) [25842] STRUCT ahd_tmode_lstate (0 bytes) [28371] STRUCT lock_class_key (0 bytes) [36464] STRUCT radeon_legacy_backlight_privdata (0 bytes) [46237] STRUCT acx_opns (0 bytes) [46940] STRUCT mm_struct (0 bytes)
