Coqprime : fix generator for ocaml >= 5

[thery]

Fix the problem with Big_int 82

[palmskog]

@thery we still get this error log, could there be a missing dependency on ocamlfind?

# make: ocamlfind: No such file or directory
# ocamlc -o o2v -I +str  zarith.cma str.cma parser.ml
# File "parser.ml", line 3, characters 5-14:
# 3 | open Big_int_Z
#          ^^^^^^^^^
# Error: Unbound module Big_int_Z