Use proper GetDatum function in pg_stat_get_slru().

This commit changes pg_stat_get_slru() so that it uses
TimestampTzGetDatum() for stats_reset field because that field
stores the timestamp with time zone value. Previously
Int64GetDatum() was used.

Author: Fujii Masao
Reviewed-by: Tomas Vondra
Discussion: https://postgr.es/m/b8784fe6-1401-ab35-aa14-d57b5bb8e312@oss.nttdata.com

src/backend/utils/adt/pgstatfuncs.c

@@ -1757,7 +1757,7 @@ pg_stat_get_slru(PG_FUNCTION_ARGS)
 		values[5] = Int64GetDatum(stat.blocks_exists);
 		values[6] = Int64GetDatum(stat.flush);
 		values[7] = Int64GetDatum(stat.truncate);
-		values[8] = Int64GetDatum(stat.stat_reset_timestamp);
+		values[8] = TimestampTzGetDatum(stat.stat_reset_timestamp);
 
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
 	}