Fix iOS import of std::fs

[will3942]

Otherwise a build for iOS fails with the following error:

error[E0433]: failed to resolve: use of undeclared crate or module `fs`
   --> /Users/willevans/.cargo/registry/src/index.crates.io-6f17d22bba15001f/souvlaki-0.8.1/src/platform/macos/mod.rs:293:22
    |
293 |     let image_data = fs::read(&url).unwrap();
    |                      ^^ use of undeclared crate or module `fs`

[Sinono3]

Thank you! Merging.