memcg: make v1 only functions static

The functions memcg1_charge_statistics() and memcg1_check_events() are
never used outside of v1 source file.  So, make them static.

Link: https://lkml.kernel.org/r/20240815050453.1298138-6-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>