Restrict leveldb.1.3.0 from building on OCaml 5
FTBFS due to C API renames:
#=== ERROR while compiling leveldb.1.3.0 ======================================#
# 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/leveldb.1.3.0
# command ~/.opam/opam-init/hooks/sandbox.sh build dune build -p leveldb -j 47 @install
# exit-code 1
# env-file ~/.opam/log/leveldb-8-8eb1b9.env
# output-file ~/.opam/log/leveldb-8-8eb1b9.out
### output ###
# File "src/dune", line 6, characters 9-22:
# 6 | (names leveldb_stubs))
# ^^^^^^^^^^^^^
# (cd _build/default/src && /usr/bin/gcc -O2 -fno-strict-aliasing -fwrapv -pthread -fPIC -g -I /home/opam/.opam/5.0/lib/ocaml -I /home/opam/.opam/5.0/lib/ocaml/threads -I /home/opam/.opam/5.0/lib/ocaml/unix -o leveldb_stubs.o -c leveldb_stubs.cc)
# leveldb_stubs.cc: In function 'value ldb_destroy(value)':
# leveldb_stubs.cc:376:32: error: 'string_length' was not declared in this scope; did you mean 'caml_string_length'?
# 376 | std::string _s(String_val(s), string_length(s));
# | ^~~~~~~~~~~~~
# | caml_string_length