ocaml5: restrict cairo2
It uses unprefixed API:
#=== ERROR while compiling cairo2.0.6.3 =======================================#
# 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/cairo2.0.6.3
# command ~/.opam/opam-init/hooks/sandbox.sh build dune runtest -p cairo2 -j 255
# exit-code 1
# env-file ~/.opam/log/cairo2-7-252ab9.env
# output-file ~/.opam/log/cairo2-7-252ab9.out
### output ###
# File "tests/dune", line 3, characters 21-31:
# 3 | (names image_create matrix_set surface_gc test_for_stream
# ^^^^^^^^^^
# (cd _build/default && /home/opam/.opam/5.0/bin/ocamlopt.opt -w -40 -g -o tests/matrix_set.exe src/cairo.cmxa -I src tests/.image_create.eobjs/native/matrix_set.cmx)
# /usr/bin/ld: src/libcairo_stubs.a(cairo_stubs.o): in function `caml_cairo_create':
# /home/opam/.opam/5.0/.opam-switch/build/cairo2.0.6.3/_build/default/src/cairo_stubs.c:53: undefined reference to `alloc_custom'