Fix thinko in previous commit
Fix thinko in previous commit

Since postgres.h includes palloc.h, definitions that affect the latter
must be present before the former is included.

Per buildfarm results

ashwinstar/gpdb,ovr/postgres-xl,lisakowen/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,adam8157/gpdb,yazun/postgres-xl,techdragon/Postgres-XL,oberstet/postgres-xl,greenplum-db/gpdb,zeroae/postgres-xl,xinzweb/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,Postgres-XL/Postgres-XL,lisakowen/gpdb,50wu/gpdb,ovr/postgres-xl,50wu/gpdb,adam8157/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,lisakowen/gpdb,yazun/postgres-xl,ashwinstar/gpdb,jmcatamney/gpdb,adam8157/gpdb,techdragon/Postgres-XL,zeroae/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,lisakowen/gpdb,yazun/postgres-xl,50wu/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,ovr/postgres-xl,lisakowen/gpdb,xinzweb/gpdb,ovr/postgres-xl,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,zeroae/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,pavanvd/postgres-xl,pavanvd/postgres-xl,yazun/postgres-xl,techdragon/Postgres-XL,ashwinstar/gpdb,50wu/gpdb,ovr/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,yazun/postgres-xl,techdragon/Postgres-XL,oberstet/postgres-xl,xinzweb/gpdb,techdragon/Postgres-XL