Add the C client include directory

Will eventually need to move the `aeron/` folder and add an
aeron-client-sys crate.

aeron-driver-sys/build.rs

@@ -91,6 +91,7 @@ pub fn main() {
     println!("cargo:include={}", header_path.display());
     let bindings = bindgen::Builder::default()
         .clang_arg(format!("-I{}", header_path.display()))
+        .clang_arg(format!("-I{}", aeron_path.join("aeron-client/src/main/c").display()))
         .header("bindings.h")
         .whitelist_function("aeron_.*")
         .whitelist_type("aeron_.*")