Require posix_ipc for lockutils

Nova commit 01b208a06c6841b7e03e2a8657941cad1947b453 brought
oslo-incubator commit edce46cf5efd6d738d379205f9bf526a498845e3 over to
nova but didn't update nova's requirements.txt to require posix_ipc,
which is required for that code in lockutils.

Closes-Bug: #1338644

Change-Id: Ic6bb599b31436732cfa71a17272777bace8dfc88