Fix embedding with MSVC (#60142)
The use of `__attribute__` in headers causes compile errors on MSVC,
this PR removes these when compiling a program that includes Julia
headers with MSVC.
The `DECLSPEC_IMPORT` macro is also defined by default on MSVC, so it is
deactivated when in MSVC.
See the [log starting
here](https://github.com/JuliaInterop/libcxxwrap-julia/actions/runs/19379560768/job/55455166299#step:4:60)
for an example of the errors.
