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.
Perhaps it should. I wanted to avoid changing the existing MinGW build too dramatically, where either we have a `-Wl,--export-all-symbols` somewhere or MinGW is just defaulting to exporting everything if there are no explicit dllexport instructions provided. If we make this definition apply for MinGW as well, then any function other than these 4 (which look to be the only ones currently `ccall`ed somewhere in base Julia) will no longer be exported from the DLL.

In some ways the `__declspec(dllexport)` annotation is like saying "this is part of the public API if this library gets linked into a DLL." I don't know enough about utf8proc to draw that line beyond the 4 functions that are used in base Julia code (not sure about anything in `src/` for example) - should everything be exported?