/usr/lib/ocaml/stublibs/dllpolkaMPQ_caml.so is in libapron-ocaml 0.9.10-9+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ocaml/stublibs/libpolkaMPQ_caml.so
/usr/lib/ocaml/stublibs/libpolkaMPQ_caml.so