Depend on ECJPAKE key exchange, not module

This is more consistent, as it doesn't make any sense for a user to be able to
set up an EC J-PAKE password with TLS if the corresponding key exchange is
disabled.

Arguably this is what we should de for other key exchanges as well instead of
depending on ECDH_C etc, but this is an independent issue, so let's just do
the right thing with the new key exchange and fix the other ones later. (This
is a marginal issue anyway, since people who disable all ECDH key exchange are
likely to also disable ECDH_C in order to minimize footprint.)

include/mbedtls/ssl.h

@@ -1676,7 +1676,7 @@ void mbedtls_ssl_conf_sni( mbedtls_ssl_config *conf,
                   void *p_sni );
 #endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */
 
-#if defined(MBEDTLS_ECJPAKE_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
 /**
  * \brief          Set the EC J-PAKE password for current handshake.
  *
@@ -1697,7 +1697,7 @@ void mbedtls_ssl_conf_sni( mbedtls_ssl_config *conf,
 int mbedtls_ssl_set_hs_ecjpake_password( mbedtls_ssl_context *ssl,
                                          const unsigned char *pw,
                                          size_t pw_len );
-#endif /*MBEDTLS_ECJPAKE_C */
+#endif /*MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
 
 #if defined(MBEDTLS_SSL_ALPN)
 /**