ldb:dlinklist: avoid -Wtautological-compare errors with gcc6

ldb:dlinklist: avoid -Wtautological-compare errors with gcc6

We expect these macros to generate tautological compares
intentionally, so disabling the warning is just fine.

This lets --pick-developer work with gcc6 and newer.

Pair-Programmed-With: Michael Adam <obnox@samba.org>

Signed-off-by: Michael Adam <obnox@samba.org>
Signed-off-by: Ira Cooper <ira@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Jul 14 05:44:21 CEST 2016 on sn-devel-144