fixed dllcore_kernel_stubs undefined symbols on OpenBSD

/home/lsd/.opam/4.01.0/bin/ocamlrun:/home/lsd/.opam/4.01.0/lib/stublibs/dllcore_kernel_stubs.so: undefined symbol 'bswap16'
/home/lsd/.opam/4.01.0/bin/ocamlrun:/home/lsd/.opam/4.01.0/lib/stublibs/dllcore_kernel_stubs.so: undefined symbol 'bswap64'
/home/lsd/.opam/4.01.0/bin/ocamlrun:/home/lsd/.opam/4.01.0/lib/stublibs/dllcore_kernel_stubs.so: undefined symbol 'bswap32'
Cannot load required shared library dllcore_kernel_stubs.
Reason: /home/lsd/.opam/4.01.0/lib/stublibs/dllcore_kernel_stubs.so: Cannot load specified object.