wrap sql *.c files with #ifdef SLAPD_SQL to facilate NT builds

servers/slapd/back-sql/bind.c

servers/slapd/back-sql/config.c

servers/slapd/back-sql/entry-id.c

servers/slapd/back-sql/init.c

servers/slapd/back-sql/modify.c

servers/slapd/back-sql/other.c

servers/slapd/back-sql/schema-map.c

servers/slapd/back-sql/search.c

servers/slapd/back-sql/sql-wrap.c

servers/slapd/back-sql/util.c