Remove __declspec(dllimport) from Windows libiconv build

[HertzDevil]

The libiconv static library we build for our Windows packages apparently tries to import libiconv_version from the libiconv DLL, so almost every program built on Windows behaves like a DLL that depends on libiconv, producing messages similar to the following during compilation:

Creating library C:\Users\runneradmin\AppData\Local\crystal\cache\D-a-crystal-crystal-src-ecr-process.cr\macro_run.lib and object C:\Users\runneradmin\AppData\Local\crystal\cache\D-a-crystal-crystal-src-ecr-process.cr\macro_run.exp
Creating library D:\a\crystal\crystal\.build\crystal-next.lib and object D:\a\crystal\crystal\.build\crystal-next.exp

This PR fixes it by removing this hardcoded __declspec(dllimport) added when that third-party repository upgraded libiconv from 1.16 to 1.17, introduced via #12745. (This __declspec is never a good idea in the first place because MSVC would warn about conflicting keywords when building a libiconv DLL.)