xen-evtchn: update to 1.0.3

Unfortunately 1.0.2 was a bit of a turkey: it wasn't possible to
link ocaml-vchan correctly.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #2235 from djs55/evtchn.1.0.2

xen-evtchn: update to 1.0.3