diffie-hellman: Use bool instead of status_t as get_shared_secret() return value

While such a change is not unproblematic, keeping status_t makes the API
inconsistent once we introduce return values for the public value operations.

src/libstrongswan/plugins/gcrypt/gcrypt_dh.c

@@ -138,15 +138,15 @@ METHOD(diffie_hellman_t, get_my_public_value, void,
 	*value = export_mpi(this->ya, this->p_len);
 }
 
-METHOD(diffie_hellman_t, get_shared_secret, status_t,
+METHOD(diffie_hellman_t, get_shared_secret, bool,
 	private_gcrypt_dh_t *this, chunk_t *secret)
 {
 	if (!this->zz)
 	{
-		return FAILED;
+		return FALSE;
 	}
 	*secret = export_mpi(this->zz, this->p_len);
-	return SUCCESS;
+	return TRUE;
 }
 
 METHOD(diffie_hellman_t, get_dh_group, diffie_hellman_group_t,