doc: Add missing include in example code
doc: Add missing include in example code

It's not broken because the header file is included via other headers,
but for better style we should be more explicit.

Reported-by: d4388d1d0baca44773bdec42ed589515a83070a2@hme.com

xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb