[OCaml] Install .mli (interface) files

Install the OCaml interface .mli files. Those files were most likely
omitted because they are input files for the compiled .cmi files.
However, installing them is reasonable since -- unlike .cmi files --
they are human-readable.

The issue was originally spotted by @jpdeplaix.

Differential Revision: https://reviews.llvm.org/D25128

llvm-svn: 283028