ios/example01 does not build in Xcode 12, fails with a linker error for _main_rs

[coolbluewater]

Hi, thanks a lot for this example! It's very well thought out and links Xcode and Rust very elegantly.

It is in need of an update, however. Xcode 12 does not have the Single View App project type and various other settings have been moved around.

Also, the project does not build with Xcode 12. The link step fails, with ld unable to resolve _main_rs. Xcode 10 is able to build the exact same project with no issues. I've got both Xcode 10 and Xcode 12 installed side by side and so can go back and forth between them.

Any idea what might have changed between Xcode 10 and Xcode 12?