ldap: fix filter property

Acked-by: Karsten Schwank
Target: master, 2.6, 2,7
Require-book: no
Require-notes: no
(cherry picked from commit 635005642d016254f449a81a72cdc739465cfa62)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Merge pull request #307 from kofemann/fix-ldap-2.7

ldap: fix filter property