Autogenerate include/updater.h every time we build. (#3)

Turns out this was way easier than I thought.
https://github.com/shorebirdtech/shorebird/issues/121
Docs:
https://github.com/eqrion/cbindgen/blob/master/docs.md#buildrs

library/Cargo.toml

@@ -48,3 +48,7 @@ log-panics = { version = "2", features = ["with-backtrace"]}
 
 [dev-dependencies]
 tempdir = "0.3.7"
+
+# https://github.com/eqrion/cbindgen/blob/master/docs.md#buildrs
+[build-dependencies]
+cbindgen = "0.24.0"

library/build.rs

@@ -0,0 +1,13 @@
+extern crate cbindgen;
+
+use std::env;
+
+// See https://github.com/eqrion/cbindgen/blob/master/docs.md#buildrs
+fn main() {
+    let crate_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
+
+    // Should this write to the out dir (target) instead?
+    cbindgen::generate(crate_dir)
+        .expect("Unable to generate bindings")
+        .write_to_file("include/updater.h");
+}