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/ostree/ot-builtin-cat.c

@@ -24,6 +24,7 @@
 
 #include "ot-builtins.h"
 #include "ostree.h"
+#include "otutil.h"
 
 #include <gio/gunixoutputstream.h>