Avoid rebuilds triggered by build.rs unless src/lib.rs actually changed.

Author: kinetiknz

mp4parse_capi/build.rs

@@ -1,6 +1,7 @@
 extern crate cheddar;
 
 fn main() {
+    println!("cargo:rerun-if-changed=src/lib.rs");
     // Generate mp4parse.h.
     cheddar::Cheddar::new().expect("could not read manifest")
         .insert_code("// THIS FILE IS AUTOGENERATED BY mp4parse-rust/build.rs - DO NOT EDIT\n\n")