Ignore Linux-only Rustdoc test on macOS

Due to incorrect CI configuration, this test was not being run on
macOS. aarch64-apple-darwin will start running it, so we deliberately
ignore it.

Author: shepmaster

tests/rustdoc-ui/issues/issue-107918.rs

@@ -1,6 +1,7 @@
 // aux-build:panic-handler.rs
 // compile-flags: --document-private-items
 // build-pass
+// ignore-macos
 // ignore-windows
 
 #![no_std]