pgsql: We need to rebuild objfiles.txt when one of the subdirectories'

Log Message:
-----------
We need to rebuild objfiles.txt when one of the subdirectories' objfiles.txt
changed in case a new file got added.

Modified Files:
--------------
pgsql/src/backend:
common.mk (r1.6 -> r1.7)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/common.mk?r1=1.6&r2=1.7)