ocaml-xenstore: Mark as conflicting with upstream xen-ocaml package

For now, at least, we need to use our own build of xenstore, not
the version provided by upstream.

Reported-by: Sayid Munawar
Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #153 from euanh/build-improvements

ocaml-xenstore: Mark as conflicting with upstream xen-ocaml package