tools/ocaml: Fix caml-stubdom make line to depend on the ocaml cross compiler

tools/ocaml: Fix caml-stubdom make line to depend on the ocaml cross compiler

The provided sample caml stubdom needs to use the ocaml cross
compiler, but if it hasn't been downloaded and compiled, then it won't
find it and the build will fail.  This makes caml-stubdom depend on
the cross compiler, so it will automatically get built before the
stubdom.

Signed-off-by: Patrick Colp <pjcolp@cs.ubc.ca>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>