lib: use ostree-content-writer header

This installs and exposes the content of `ostree-content-writer.h`,
so that library consumers can properly reference symbols defined
in that header.

src/libostree/ostree.h

@@ -20,6 +20,7 @@
 #pragma once
 
 #include <ostree-async-progress.h>
+#include <ostree-content-writer.h>
 #include <ostree-core.h>
 #include <ostree-repo.h>
 #include <ostree-repo-os.h>