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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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/aeb6f4b3b0d079155f9129ff6b043e9c8a926e72

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