Use SR_API to mark public API symbols, and SR_PRIV for private symbols.

Variables and functions marked 'static' are private already and don't
need SR_PRIV. However, functions which are not static (because they need
to be used in other libsigrok-internal files) but are also not meant to
be part of the public libsigrok API, must use SR_PRIV.

This uses the 'visibility' feature of gcc (requires gcc >= 4.0).

Details: http://gcc.gnu.org/wiki/Visibility