darwin.libiconv: use static library

This is necessary to get stuff into the Apple App Store for iOS. Apple does not really like using dynamic libraries in iOS. Also the libcharset thing is apparently a "private" symbol.

See https://github.com/reflex-frp/reflex-platform/pull/307 for more details.

See https://github.com/NixOS/nixpkgs/commit/3dee596ed1e373c296cf4d5a09b937b41f27fbe5#diff-261d74054a31954b7c6175e2e7e28356 for discussion