pgsql: Fix map_sql_table_to_xmlschema() with dropped attributes.

Log Message:
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Fix map_sql_table_to_xmlschema() with dropped attributes.

also backpatched to 8.3

Modified Files:
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pgsql/src/backend/utils/adt:
xml.c (r1.88 -> r1.89)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c?r1=1.88&r2=1.89)