owl-base: add upper bound on ocaml 5
Parts of the library do work fine, however
some modules in owl-base (Owl_log, Owl_io and Owl_utils)
use the Unix module without linking against it,
making packages fail in unexpected ways.
For example
```
=== ERROR while compiling owl-ode-odepack.0.4.0 ==============================#
context 2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
path ~/.opam/5.0/.opam-switch/build/owl-ode-odepack.0.4.0
command ~/.opam/opam-init/hooks/sandbox.sh build dune runtest -p owl-ode-odepack -j 255
exit-code 1
env-file ~/.opam/log/owl-ode-odepack-7-f6b113.env
output-file ~/.opam/log/owl-ode-odepack-7-f6b113.out
\## output ###
(cd _build/default && /home/opam/.opam/5.0/bin/ocamlopt.opt -w -40 -g -o examples/van_der_pol_odepack.exe /home/opam/.opam/5.0/lib/bigarray-compat/bigarray_compat.cmxa /home/opam/.opam/5.0/lib/stdlib-shims/stdlib_shims.cmxa /home/opam/.opam/5.0/lib/integers/integers.cmxa -I /home/opam/.opam/5.0/lib/integers /home/opam/.opam/5.0/lib/ctypes/ctypes.cmxa -I /home/opam/.opam/5.0/lib/ctypes /home/opam/.opam/5.0/lib/ocaml/str/str.cmxa /home/opam/.opam/5.0/lib/ctypes/cstubs.cmxa -I /home/opam/.opam/5.0/lib/ctypes /home/opam/.opam/5.0/lib/eigen/cpp/eigen_cpp_stubs.cmxa -I /home/opam/.opam/5.0/lib/eigen/cpp /home/opam/.opam/5.0/lib/eigen/eigen.cmxa -I /home/opam/.opam/5.0/lib/eigen /home/opam/.opam/5.0/lib/owl-base/owl_base.cmxa /home/opam/.opam/5.0/lib/ocaml/unix/unix.cmxa /home/opam/.opam/5.0/lib/zip/zip.cmxa -I /home/opam/.opam/5.0/lib/zip /home/opam/.opam/5.0/lib/npy/npy.cmxa /home/opam/.opam/5.0/lib/owl/owl.cmxa -I /home/opam/.opam/5.0/lib/owl /home/opam/.opam/5.0/lib/owl-ode-base/owl_ode_base.cmxa /home/opam/.opam/5.0/lib/owl-ode/owl_ode.cmxa /home/opam/.opam/5.0/lib/odepack/fortran/odepack_fortran.cmxa -I /home/opam/.opam/5.0/lib/odepack/fortran /home/opam/.opam/5.0/lib/odepack/odepack.cmxa -I /home/opam/.opam/5.0/lib/odepack src/odepack/owl_ode_odepack.cmxa /home/opam/.opam/5.0/lib/plplot/plplot.cmxa -I /home/opam/.opam/5.0/lib/plplot /home/opam/.opam/5.0/lib/owl-plplot/owl_plplot.cmxa examples/.van_der_pol_odepack.eobjs/native/dune__exe__Van_der_pol_odepack.cmx)
File "_none_", line 1:
Error: No implementations provided for the following modules:
Unix referenced from /home/opam/.opam/5.0/lib/owl-base/owl_base.cmxa(Owl_utils),
/home/opam/.opam/5.0/lib/owl-base/owl_base.cmxa(Owl_log),
/home/opam/.opam/5.0/lib/owl-base/owl_base.cmxa(Owl_io)
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>