RFC: add DLLEXPORT to utf8proc_get_property

ref https://github.com/JuliaLang/julia/pull/7761#issuecomment-56255111

This is needed to finish building the system image when using MSVC to build Julia. More of the API will probably need to be DLLEXPORT'ed to pass any Julia tests that use other functions.
It would probably be better to use something like `_MOJIBAKE_EXPORTS` here to avoid accidentally clashing with user-defined macros.  (It is not too far-fetched to imagine `mojibake.h` being `#included` in a project that defines a `LIBRARY_EXPORTS` macro.)