pgsql: Avoid including utils/timestamp.h in conflict.h.

Avoid including utils/timestamp.h in conflict.h.

conflict.h currently includes utils/timestamp.h despite only requiring
basic timestamp type definitions. This creates unnecessary overhead.

Replace the include with datatype/timestamp.h to provide the necessary
types. This change requires explicitly including utils/timestamp.h in
test_custom_fixed_stats.c, which previously relied on the indirect
inclusion.

Extracted from the larger patch by Andres Freund.
Discussion: https://postgr.es/m/aY-UE-4t7FiYgH3t@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/308622edf1743aa407df0044cc4529aed8de15e6

Modified Files
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src/include/replication/conflict.h | 2 +-
src/test/modules/test_custom_stats/test_custom_fixed_stats.c | 1 +
2 files changed, 2 insertions(+), 1 deletion(-)