libsrt: Explicitly control symbol visibility
Instead of having symbols visible by default (which included compatibility symbols for older versions of GLib), hide them by default (equivalent to G_GNUC_INTERNAL) and export only what we need.
This means we export the public API, plus symbols that are used by the unit tests or by the command-line diagnostic tools. Symbols used only by pressure-vessel do not need to be exported, since it links libsrt statically.
/cc @denittis