opam: pin to mirage-flow.1.1.0
opam: pin to mirage-flow.1.1.0

Version 1.2.0 removes `Mirage_flow_s` which we require.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit