librbd: Only public API symbols from the shared library

The librbd shared library was previously exporting all
symbols.  librbd public API methods are now explicitly
exported and all other symbols are hidden.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
If we aren't using LOCAL do we need to define it?

And if there's just a single #define here, should we just stick it in librbd.h and avoid a new header file?