pgsql: pg_stat_statements: Clean up REGRESS list in Makefile

pg_stat_statements: Clean up REGRESS list in Makefile

The "wal" and "entry_timestamp" items were still on the same line, which
was not intentional.

Thinko in f9afd56218af.

Reported-by: Man Zeng <zengman@halodbtech.com>
Discussion: https://postgr.es/m/aW6_Xc8auuu5iAPi@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/905ef401d5e068a1e6d246f9d96435c9d750c8be

Modified Files
--------------
contrib/pg_stat_statements/Makefile | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)