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

src/backend/access/transam/slru.c

@@ -1602,7 +1602,9 @@ SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path)
 	if (fd < 0)
 		return -1;
 
+	pgstat_report_wait_start(WAIT_EVENT_SLRU_FLUSH_SYNC);
 	result = pg_fsync(fd);
+	pgstat_report_wait_end();
 	save_errno = errno;
 
 	CloseTransientFile(fd);