Add oclock to message_switch dependencies

Signed-off-by: John Else <john.else@citrix.com>
xen-api-client needs cohttp-0.9.7

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #2 from johnelse/message-switch-oclock-dep

Add oclock to message_switch dependencies