poly1305: Don't export generic Poly1305_* symbols from xform_poly1305.c.

There currently isn't a need to provide a public interface to a
software Poly1305 implementation beyond what is already available via
libsodium's APIs and these symbols conflict with symbols shared within
the ossl.ko module between ossl_poly1305.c and ossl_chacha20.c.

Reported by: se, kp
Fixes: 78991a93eb9d
Sponsored by: Netflix