Fix shared library build.

poly1305.h was missing exports. While here, chacha.h should also be exported.

Change-Id: I5da9c953d3e5a5ef76a3e96bc4794192abee3ae6
Reviewed-on: https://boringssl-review.googlesource.com/5420
Reviewed-by: Adam Langley <agl@google.com>