Add missing const qualifier

[madroach]

This happens on current OpenBSD.

      ocamlc src/open.o
src/unix/open.c:375:25: warning: assigning to 'value *' (aka 'long *') from 'const value *' (aka 'const long *') discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
    graphic_failure_exn = caml_named_value("Graphics.Graphic_failure");
                        ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
  ocamlmklib src/dllgraphics_stubs.so,src/libgraphics_stubs.a (exit 2)
(cd _build/default && /usr/local/bin/ocamlmklib.opt -g -o src/graphics_stubs src/color.o src/draw.o src/dump_img.o src/events.o src/fill.o src/image.o src/make_img.o src/open.o src/point_col.o src/sound.o src/subwindow.o src/text.o -lX11)
ld: error: unable to find library -lX11
cc: error: linker command failed with exit code 1 (use -v to see invocation)

[dra27]

Thanks!