pgstat: prevent fix pgstat_reinit_entry() from zeroing out lwlock.

pgstat: prevent fix pgstat_reinit_entry() from zeroing out lwlock.

Zeroing out an lwlock in a normal build turns out to not trigger any alarms,
if nobody can use the lwlock at that moment (as the case here). But with
--disable-spinlocks --disable-atomics, the sema field needs to be initialized.

We probably should make sure that this fails on more common configurations as
well...

Per buildfarm animal rorqual