Minijail: remove libsyscalls.gen.o from test-clean target.
Minijail: remove libsyscalls.gen.o from test-clean target.

CFLAGS testing change does not affect libsyscalls.gen.o,
and this way we can enable the 'tests' target in the
Minijail ebuild, instead of building the unittest targets
separately.

BUG=None
TEST=cros_workon make chromeos-minijail --test, tests build and run.

Change-Id: If49d74d2db77698d9613b801661f8f75bc29d7be
Reviewed-on: https://gerrit.chromium.org/gerrit/32031
Tested-by: Jorge Lucangeli Obes <bc323d654c912a6c99763475eeb77a04da410a4a@chromium.org>
Reviewed-by: Will Drewry <72416dde44ea762d19e3a7a555e3b59ce98f8eea@chromium.org>
Commit-Ready: Jorge Lucangeli Obes <bc323d654c912a6c99763475eeb77a04da410a4a@chromium.org>

omegaup/minijail,omegaup/minijail,omegaup/minijail,omegaup/minijail