Mark PKey as `Send` and `Sync`

Provided that the locking function is set, the underlying `ffi::EVP_KEY`
type should be safe to use across threads.

openssl/src/crypto/pkey.rs

@@ -57,6 +57,9 @@ pub struct PKey {
     parts: Parts,
 }
 
+unsafe impl Send for PKey {}
+unsafe impl Sync for PKey {}
+
 /// Represents a public key, optionally with a private key attached.
 impl PKey {
     pub fn new() -> PKey {