Expose curves for ECDH

Change-Id: Ifed7917ff1f54f2fbacf9abb967465d921fd7e3e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63045
Reviewed-by: Adam Langley <agl@google.com>

rust/bssl-crypto/src/ecdh.rs

@@ -21,6 +21,8 @@ use crate::{
     CSliceMut, ForeignType,
 };
 
+pub use crate::ec::P256;
+
 /// Private key used in a elliptic curve Diffie-Hellman.
 pub struct PrivateKey<C: Curve> {
     /// An EcKey containing the private-public key pair