mm: vm_event_item: explicit #include for THREAD_SIZE

This header uses THREAD_SIZE, which is provided by the thread_info.h
header but is not included in this header.  Depending on the #include
ordering in other files, this can produce preprocessor errors.

Link: https://lkml.kernel.org/r/20250909201419.827638-1-briannorris@chromium.org


Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>