Rollup merge of rust-lang#74520 - RalfJung:backtrace-src, r=Mark-Simulacrum

include backtrace folder in rust-src component

libstd has a [mandatory dependency on this code](https://github.com/rust-lang/rust/pull/73441/files#diff-242481015141f373dcb178e93cffa850), ergo we need to include it in rust-src.

r? @oli-obk
Fixes rust-lang#74506

Author: Manishearth

src/bootstrap/dist.rs

@@ -1005,6 +1005,7 @@ impl Step for Src {
         // (essentially libstd and all of its path dependencies)
         let std_src_dirs = [
             "src/build_helper",
+            "src/backtrace",
             "src/liballoc",
             "src/libcore",
             "src/libpanic_abort",