Add dummy |SSL_get_server_tmp_key|.

Node.js calls it but handles it failing. Since we have abstracted this
in the state machine, we mightn't even be using a cipher suite where the
server's key can be expressed as an EVP_PKEY.

Change-Id: Ic3f013dc9bcd7170a9eb2c7535378d478b985849
Reviewed-on: https://boringssl-review.googlesource.com/7272
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/ssl.h

diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index a21b6ee2..7cc901b 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -3148,6 +3148,9 @@
 /* SSL_get_current_expansion returns NULL. */
 OPENSSL_EXPORT const COMP_METHOD *SSL_get_current_expansion(SSL *s);
 
+/* SSL_get_server_tmp_key returns zero. */
+OPENSSL_EXPORT int *SSL_get_server_tmp_key(SSL *ssl, EVP_PKEY **out_key);
+
 #define SSL_set_app_data(s, arg) (SSL_set_ex_data(s, 0, (char *)arg))
 #define SSL_get_app_data(s) (SSL_get_ex_data(s, 0))
 #define SSL_SESSION_set_app_data(s, a) \