pgsql: Re-add tracking of wait event SLRUFlushSync

Re-add tracking of wait event SLRUFlushSync

SLRUFlushSync has been accidently removed during dee663f, that has moved
the flush of the SLRU files to the checkpointer, so add it back.  The
issue has been noticed by Thomas when checking for orphaned wait
events.

Author: Thomas Munro
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/CA+hUKGK6tqm59KuF1z+h5Y8fsWcu5v8+84kduSHwRzwjB2aa_A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/84cc1426745b4bc91242caec9173a2ca19a4af79

Modified Files
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src/backend/access/transam/slru.c | 2 ++
1 file changed, 2 insertions(+)