The basic rule we follow here is to always first include 'postgres.h' or
'postgres_fe.h' whichever is applicable, then system header includes and
then Postgres header includes.  In this, we also follow that all the
Postgres header includes are in order based on their ASCII value.  We
generally follow these rules, but the code has deviated in many places.
This commit makes it consistent just for contrib modules.  The later
commits will enforce similar rules in other parts of code.

Author: Vignesh C
Reviewed-by: Amit Kapila
Discussion: https://postgr.es/m/CALDaNm2Sznv8RR6Ex-iJO6xAdsxgWhCoETkaYX=+9DW3q0QCfA@mail.gmail.com