Fix rust-src component.

Author: ehuss

src/bootstrap/dist.rs

@@ -1005,7 +1005,7 @@ impl Step for Src {
         // (essentially libstd and all of its path dependencies)
         let std_src_dirs = [
             "src/build_helper",
-            "src/backtrace",
+            "src/backtrace/src",
             "src/liballoc",
             "src/libcore",
             "src/libpanic_abort",