Add support for the psk_key_exchange_modes extension

This is required for the later addition of resumption support.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2259)

ssl/ssl_err.c

diff --git a/ssl/ssl_err.c b/ssl/ssl_err.c
index 099af01..2d9401a 100644
--- a/ssl/ssl_err.c
+++ b/ssl/ssl_err.c
@@ -300,6 +300,8 @@
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_CTOS_NPN), "tls_construct_ctos_npn"},
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_CTOS_PADDING),
      "tls_construct_ctos_padding"},
+    {ERR_FUNC(SSL_F_TLS_CONSTRUCT_CTOS_PSK_KEX_MODES),
+     "tls_construct_ctos_psk_kex_modes"},
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_CTOS_RENEGOTIATE),
      "tls_construct_ctos_renegotiate"},
     {ERR_FUNC(SSL_F_TLS_CONSTRUCT_CTOS_SCT), "tls_construct_ctos_sct"},