crypto: x86/poly1305 - fix .gitignore typo

Admist the kbuild robot induced changes, the .gitignore file for the
generated file wasn't updated with the non-clashing filename. This
commit adjusts that.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>