Clean up Makefile.win32 "-I" flag additions.
Clean up Makefile.win32 "-I" flag additions.

The PGXS-case directory does not exist in the non-PGXS case, and vice
versa.  Add one or the other, not both.  This is essentially cosmetic.
It makes Makefile.win32 more like the similar Makefile.global code.

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