Avoid race condition

There seems to be some race-condition in build.rs
that causes random build results to be produced for `/usr/bin/difft`.

The change seems to have negligible influence on overall build time.

build.rs

@@ -249,7 +249,7 @@ fn main() {
         println!("cargo:rerun-if-changed={}", parser.src_dir);
     }
 
-    parsers.par_iter().for_each(|p| p.build());
+    parsers.iter().for_each(|p| p.build());
     commit_info();
 
     if let Some((version, _, _)) = rustc::triple() {