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-enumtypes.h.template

@@ -33,6 +33,7 @@ G_BEGIN_DECLS
 
 /*** BEGIN enumeration-production ***/
 #define @ENUMPREFIX@_TYPE_@ENUMSHORT@ (@enum_name@_get_type ())
+_OSTREE_PUBLIC
 GType @enum_name@_get_type (void) G_GNUC_CONST;
 
 /*** END enumeration-production ***/