Add rerun-if-changed to fiber/build.rs (#4377)

Not having rerun-if-changed leads to including nanosecond-precision
mtimes in fingerprints and unnecessary rebuilds in docker [1].

[1] https://github.com/rust-lang/cargo/blob/0.63.1/src/cargo/core/compiler/fingerprint.rs#L1491

crates/fiber/build.rs

@@ -5,13 +5,16 @@ fn main() {
     let arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap();
     let os = env::var("CARGO_CFG_TARGET_OS").unwrap();
     if os == "windows" {
+        println!("cargo:rerun-if-changed=src/windows.c");
         build.file("src/windows.c");
     } else if arch == "s390x" {
+        println!("cargo:rerun-if-changed=src/unix/s390x.S");
         build.file("src/unix/s390x.S");
     } else {
         // assume that this is included via inline assembly in the crate itself,
         // and the crate will otherwise have a `compile_error!` for unsupported
         // platforms.
+        println!("cargo:rerun-if-changed=build.rs");
         return;
     }
     build.define(&format!("CFG_TARGET_OS_{}", os), None);