use wrapper for including expat

This is necessary for building on systems that don't store headers in /usr/include

build.rs

@@ -10,7 +10,7 @@ fn main()
 {
     let builder = bindgen::builder();
 
-    let bindings = builder.header("/usr/include/expat.h")
+    let bindings = builder.header("include/wrapper.h")
                           .generate()
                           .expect("The libexpat headers must be installed");