debian/control: xen-doc: add ${misc:Depends}

debian/control: xen-doc: add ${misc:Depends}

Lintian complains:

    W: xen source: debhelper-but-no-misc-depends xen-doc
    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: warning
    N:
    N:    Check: debhelper

Signed-off-by: Hans van Kranenburg <hans@knorrie.org>