trivial designated initializers

Hi

We use C99 designated struct initializers in many places, but for some
reason we don't do it in the tupleLockExtraInfo array in heapam.c nor in
InternalBGWorkers array in bgworker.c.  I've had this trivial patch
rotting in a worktree for a long time.  Any opposition to this change?

Thanks,

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Postgres is bloatware by design: it was built to house
 PhD theses." (Joey Hellerstein, SIGMOD annual conference 2002)