patch: install *.mli files

[ecc1]

I apply this patch in the Debian packaging of ocaml-sha; please consider it for the upstream version:

http://anonscm.debian.org/gitweb/?p=pkg-ocaml-maint/packages/ocaml-sha.git;a=blob_plain;f=debian/patches/0001-install-mli-files.patch;hb=HEAD

[talex5]

Hopefully this will be fixed as a side-effect of #29.