pgsql: Change expand_subsys function so that it preserves the relative

Log Message:
-----------
Change expand_subsys function so that it preserves the relative order of
the files passed as argument. This is desirable so that the dtrace rule
in src/backend/Makefile works.

Modified Files:
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pgsql/src/backend:
common.mk (r1.4 -> r1.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/common.mk?r1=1.4&r2=1.5)