Add libproc_macro to rust-src distribution

Author: vlad20012

src/bootstrap/dist.rs

@@ -853,6 +853,7 @@ 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",