Debian: Add ${misc:Depends} dependency to python-openvswitch

As reported by lintian -i:

W: openvswitch source: debhelper-but-no-misc-depends python-openvswitch
N:
N:    The source package uses debhelper, but it does not include
N:    ${misc:Depends} in the given binary package's debian/control entry.  Any
N:    debhelper command may add dependencies to ${misc:Depends} that are
N:    required for the work that it does, so recommended best practice is to
N:    always add ${misc:Depends} to the dependencies of each binary package if
N:    debhelper is in use.
N:
N:    Refer to the debhelper(7) manual page for details.
N:
N:    Severity: normal, Certainty: possible