pgsql: informix.c was violating the coding rule about not including any

Log Message:
-----------
informix.c was violating the coding rule about not including any
system headers before c.h. Per report from J6M.

Modified Files:
--------------
pgsql/src/interfaces/ecpg/compatlib:
informix.c (r1.53 -> r1.54)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/compatlib/informix.c?r1=1.53&r2=1.54)