pgsql: Use "(void)" to mark pgstat_lock_entry(..., false) calls.

Use "(void)" to mark pgstat_lock_entry(..., false) calls.

This should silence Coverity's complaints about the result being
sometimes ignored.

I'm inclined to think that these routines are simply misdesigned,
because sometimes it's okay to ignore the result and sometimes it
isn't, and we have no way to enforce the latter. But for now
I just added a comment.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/2e4ccf1b4508cc337bb4d0afff1e32a049d549fc

Modified Files
--------------
src/backend/utils/activity/pgstat.c | 2 +-
src/backend/utils/activity/pgstat_database.c | 2 +-
src/backend/utils/activity/pgstat_shmem.c | 11 ++++++++++-
3 files changed, 12 insertions(+), 3 deletions(-)