build: add symbol visibility support

Changes default visibility of symbols to hidden and makes only the API
public. This patch aims to provide not only a stable API, but a stable
ABI as well to any users of libring.

This decreases the number of exported symbols significantly, benefitting
dynamic linking time. Also decreases the chances of a symbol collision,
and enables compiler optimizations.

Bumps version to 6.0.0; clients must be recompiled.

Change-Id: I5b639a6c0933af3021e40369b2e80d9a0b825e89