lib: Introduce versioned symbols

As rpm-ostree evolves, it keeps driving API additions to libostree.
This creates a relatively tight coupling.

However, if delivering via e.g. RPM, unless one manually remembers to
increment the `Requires:` in the spec file, it's possible for the two
to become desynchronized.

RPM handles versioned symbols and will ensure a dependency if the
application starts using a newer version.

To implement this, switch to `-fvisibility=hidden`, along with an
annotation in the header, and finally add a `.sym` file.

This matches what other projects like systemd and libvirt do.

Although rather than attempting to retroactively version symbols, glom
them all onto the current one.

src/libostree/ostree-types.h

@@ -24,6 +24,10 @@
 
 #include <gio/gio.h>
 
+#ifndef _OSTREE_PUBLIC
+#define _OSTREE_PUBLIC extern
+#endif
+
 G_BEGIN_DECLS
 
 typedef struct OstreeRepo OstreeRepo;