AVRO-1358. C: Hide symbols that aren't part of the public API.

Right now we're doing this via GCC visibility pragmas.  The only symbols that
should be exported by libavro.so begin with `avro_` or `AVRO_`.


git-svn-id: https://svn.apache.org/repos/asf/avro/trunk@1551936 13f79535-47bb-0310-9956-ffa450edef68