crypto: x86/curve25519 - disable gcov

curve25519-x86_64.c fails to build when CONFIG_GCOV_KERNEL is enabled.
The error is "inline assembly requires more registers than available"
thrown from the `fsqr()` function. Therefore, excluding this file from
GCOV profiling until this issue is resolved. Thereby allowing
CONFIG_GCOV_PROFILE_ALL to be enabled for x86.

Signed-off-by: Joe Fradley <joefradley@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>