debian/control: drop autotools-dev build dependency

debian/control: drop autotools-dev build dependency

Lintian complains:

    W: xen source: useless-autoreconf-build-depends autotools-dev
    N:
    N:    Since compatibility level 10, debhelper enables the autoreconf sequence
    N:    by default.
    N:
    N:    It is therefore not necessary to specify build-dependencies on
    N:    dh-autoreconf or autotools-dev and they can be removed.
    N:
    N:    Severity: warning
    N:
    N:    Check: debhelper

So, drop it.

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