ldb:wscript: make it possible to build samba with a system ldb again

ldb:wscript: make it possible to build samba with a system ldb again

This fixes a regression in commit fcf4a891945b22dc6eccdc71fd441f1a879f556a.

If we check for 'ldb' later the 'pyldb-util' can't depend on the 'ldb' check.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11458

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Mon Aug 31 18:53:16 CEST 2015 on sn-devel-104