install utf8proc.h

This is needed by `src/flisp/julia_extensions.c`, and would be useful to have included in the binary installs.

We're currently building a static library of `libutf8proc.a`, but not installing it. That could be added as well, thoughts? The header's about 15 kb, the static library more like 550-650 kb.

(Similar story for openlibm, the static library and some of its headers are used by base Julia but not getting installed.)