pgsql: Save pg_stat_statements statistics file into $PGDATA/pg_stat dir

Save pg_stat_statements statistics file into $PGDATA/pg_stat directory at shutdown.

187492b6c2e8cafc5b39063ca3b67846e8155d24 changed pgstat.c so that
the stats files were saved into $PGDATA/pg_stat directory when the server
was shutdowned. But it accidentally forgot to change the location of
pg_stat_statements permanent stats file. This commit fixes pg_stat_statements
so that its stats file is also saved into $PGDATA/pg_stat at shutdown.

Since this fix changes the file layout, we don't back-patch it to 9.3
where this oversight was introduced.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/654e8e444749f053c3bf3fd543d10deb6aa6dd09

Modified Files
--------------
contrib/pg_stat_statements/pg_stat_statements.c | 2 +-
src/backend/postmaster/pgstat.c | 8 --------
src/include/pgstat.h | 8 ++++++++
3 files changed, 9 insertions(+), 9 deletions(-)