imagelib is not compatible with ocaml 5
it fails with multiple errors about not linking unix.
```
=== ERROR while compiling imagelib.20200929 ==================================#
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/imagelib.20200929
command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p imagelib -j 71 @install
exit-code 1
env-file ~/.opam/log/imagelib-7-975662.env
output-file ~/.opam/log/imagelib-7-975662.out
output ###
[...]
(cd _build/default && /home/opam/.opam/5.0/bin/ocamlopt.opt -w -40 -g -o app/imagetool.exe /home/opam/.opam/5.0/lib/stdlib-shims/stdlib_shims.cmxa /home/opam/.opam/5.0/lib/optint/optint.cmxa /home/opam/.opam/5.0/lib/checkseum/ocaml/checkseum_ocaml.cmxa /home/opam/.opam/5.0/lib/bigarray-compat/bigarray_compat.cmxa /home/opam/.opam/5.0/lib/decompress/de/de.cmxa /home/opam/.opam/5.0/lib/decompress/zl/zl.cmxa src/imagelib.cmxa unix/imagelib_unix.cmxa app/.imagetool.eobjs/native/dune__exe__Imagetool.cmx)
File "_none_", line 1:
Error: No implementations provided for the following modules:
Unix referenced from app/.imagetool.eobjs/native/dune__exe__Imagetool.cmx
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>