Rollup merge of rust-lang#55280 - vlad20012:add-libproc_macro-to-src-disrt, r=Mark-Simulacrum

Add libproc_macro to rust-src distribution

Fixes rust-lang#55279

Author: pietroalbini

src/bootstrap/dist.rs

@@ -881,6 +881,7 @@ impl Step for Src {
             "src/jemalloc",
             "src/libprofiler_builtins",
             "src/stdsimd",
+            "src/libproc_macro",
         ];
         let std_src_dirs_exclude = [
             "src/libcompiler_builtins/compiler-rt/test",