Add rebuild-if-changed

Incremental rebuild of `cargo check` after change in godot-core timings:

before: 48.015s average
after:  32.063s average
diff:   -33.22%

Author: lilizoey

godot-core/build.rs

@@ -16,4 +16,5 @@ fn main() {
     }
 
     godot_codegen::generate_core_files(gen_path);
+    println!("cargo:rerun-if-changed=build.rs");
 }

godot-ffi/build.rs

@@ -26,4 +26,5 @@ fn main() {
     godot_codegen::generate_sys_files(gen_path, &mut watch);
 
     watch.write_stats_to(&gen_path.join("ffi-stats.txt"));
+    println!("cargo:rerun-if-changed=build.rs");
 }