Install a shared library

This required a fair bit of surgery because previously ostree.h
included otutil.h, but that's supposed to be a private library.

src/libostree/ostree.h

@@ -22,8 +22,6 @@
 
 #pragma once
 
-#include <ostree-checksum-input-stream.h>
-#include <ostree-chain-input-stream.h>
 #include <ostree-core.h>
 #include <ostree-repo.h>
 #include <ostree-mutable-tree.h>