build: include contrib/umountd in the 'make dist' tarball

When running 'make dist' on Linux, the new contrib/umountd directory and
contents is not included in the resulting tarball. This causes the build
from the tarball to fail.

Change-Id: Ic575040212d91c6368f04b015cc9d2d5b2969a3a
BUG: 1129939
Reported-by: Emmanuel Dreyfus <manu@netbsd.org>
URL: http://supercolony.gluster.org/pipermail/gluster-devel/2014-October/042441.html
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/8907
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Emmanuel Dreyfus <manu@netbsd.org>
Tested-by: Gluster Build System <jenkins@build.gluster.com>