Makefile.nt: fix installopt to also install .mli files
Makefile.nt: fix installopt to also install .mli files

This target should install the .mli files from the middle_end,
middle_end/base_types and asmcomp directories to $(INSTALL_COMPLIBDIR),
as is done by the Unix variant of the rule.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml