Clean up #include's.

src/backend/access/gist/gist.c

src/backend/access/nbtree/nbtcompare.c

src/backend/access/rtree/rtget.c

src/backend/catalog/heap.c

src/backend/catalog/index.c

src/backend/catalog/indexing.c

src/backend/commands/cluster.c

src/backend/commands/copy.c

src/backend/commands/indexcmds.c

src/backend/executor/execUtils.c

src/backend/executor/nodeAgg.c

src/backend/optimizer/plan/createplan.c

src/backend/optimizer/plan/planner.c

src/backend/optimizer/util/plancat.c

src/backend/parser/parse_agg.c

src/backend/parser/parse_clause.c

src/backend/parser/parse_coerce.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_func.c

src/backend/parser/parse_target.c