Build fix: -lz for qemu-block xlator

LIBZ_LIBS has been changed into ZLIB_LIBS everywhere in the
sources, except in qemu-block xlator. Fix it so that it can
find -lz and link correctly.

BUG: 1129939
Change-Id: I34ded175f056d1a0898804fe602e3d2d2cba27f5
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8623
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>