make clean: fix

When running 'make clean' from the top dir, the following error happens:

 make[3]: Leaving directory `/vz/kir/git/criu/test/zdtm/live/static'
 make[2]: Leaving directory `/vz/kir/git/criu/test/zdtm/live'
 set -e; for d in lib live; do make -C $d cleanout; done
 make[2]: Entering directory `/vz/kir/git/criu/test/zdtm/lib'
 make[2]: *** No rule to make target `cleanout'.  Stop.
 make[2]: Leaving directory `/vz/kir/git/criu/test/zdtm/lib'
 make[1]: *** [cleanout] Error 2
 make[1]: Leaving directory `/vz/kir/git/criu/test/zdtm'
 make: *** [clean] Error 2

Fix by adding dummy cleanout target to test/zdtm/lib/Makefile

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>