bootstrap: x.py dist rustc-src should keep LLVM's siphash

Fixes #145117

Author: lambdageek

src/bootstrap/src/core/build_steps/dist.rs

@@ -925,9 +925,12 @@ fn copy_src_dirs(
             "llvm-project\\cmake",
             "llvm-project/runtimes",
             "llvm-project\\runtimes",
+            "llvm-project/third-party/siphash",
+            "llvm-project\\third-party\\siphash",
         ];
         if spath.contains("llvm-project")
             && !spath.ends_with("llvm-project")
+            && !(spath.ends_with("llvm-project/third-party") || spath.ends_with("llvm-project\\third-party"))
             && !LLVM_PROJECTS.iter().any(|path| spath.contains(path))
         {
             return false;