lib: Add a private copy of checksum-instream

The current `OstreeChecksumInputStream` is public due to a historical
mistake.  I'd like to add an OpenSSL checksum backend, but that's
harder without breaking this API.

Let's ignore it and create a new private version, so it's easier to do the
GLib/OpenSSL abstraction in one place.

Closes: #738
Approved by: jlebon

src/libotutil/otutil.h

@@ -53,5 +53,6 @@
 #include <ot-checksum-utils.h>
 #include <ot-gpg-utils.h>
 #include <ot-log-utils.h>
+#include <ot-checksum-instream.h>
 
 void ot_ptrarray_add_many (GPtrArray  *a, ...) G_GNUC_NULL_TERMINATED;