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
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1ed1b84bdcd26abf3c4d08a9cf1aa9f7834262ab

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