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.
@stevengj okeydokey, thanks. I'll work on that (and change the `DLLEXPORT` define to be for any `_WIN32`, and test with MinGW to make sure that doesn't break anything) in about half an hour, after the F1 race is over.