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.
Every function in the header file should be exported.