Rebuild if src changed

From the cargo docs (https://doc.rust-lang.org/cargo/reference/build-scripts.html#change-detection):

> By default, it takes a conservative approach of always re-running the build script if any file within the package is changed (or the list of files controlled by the exclude and include fields). For most cases, this is not a good choice, so it is recommended that every build script emit at least one of the rerun-if instructions (described below). If these are emitted, then Cargo will only re-run the script if the given value has changed

So, since we emit rerun-if-path-changed, we need to emit *all* the paths.
Failing to do this shows up most visibly as an outdated git version.

Fixes #11332

build.rs

@@ -49,6 +49,7 @@ fn main() {
         build_man(&targetman);
     }
     rsconf::rebuild_if_path_changed("src/libc.c");
+    rsconf::rebuild_if_path_changed("src/");
     cc::Build::new()
         .file("src/libc.c")
         .include(build_dir)