pgsql: Force synchronous_commit=on in test_decoding's concurrent_ddl_dm

Force synchronous_commit=on in test_decoding's concurrent_ddl_dml.spec.

Otherwise running installcheck-force on a server with
synchronous_commit=off will result in the tests failing. All the other
tests already do so...

Backpatch: 9.4, where logical decoding was added

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d750b20101fddefa352cf4e1ce45d6d569bda8e8

Modified Files
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contrib/test_decoding/specs/concurrent_ddl_dml.spec | 4 ++++
1 file changed, 4 insertions(+)