Mark Rustdoc test as Linux-only

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

Author: shepmaster

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

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