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.)
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 2f5ca9f..84211bb 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4932,7 +4932,7 @@
 #if defined(MBEDTLS_ECDH_C)
     mbedtls_ecdh_init( &handshake->ecdh_ctx );
 #endif
-#if defined(MBEDTLS_ECJPAKE_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
     mbedtls_ecjpake_init( &handshake->ecjpake_ctx );
 #endif
 
@@ -5451,7 +5451,7 @@
 }
 #endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */
 
-#if defined(MBEDTLS_ECJPAKE_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
 /*
  * Set EC J-PAKE password for current handshake
  */
@@ -5475,7 +5475,7 @@
                                    MBEDTLS_ECP_DP_SECP256R1,
                                    pw, pw_len ) );
 }
-#endif /* MBEDTLS_ECJPAKE_C */
+#endif /* MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
 
 #if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)
 int mbedtls_ssl_conf_psk( mbedtls_ssl_config *conf,
@@ -6619,7 +6619,7 @@
 #if defined(MBEDTLS_ECDH_C)
     mbedtls_ecdh_free( &handshake->ecdh_ctx );
 #endif
-#if defined(MBEDTLS_ECJPAKE_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
     mbedtls_ecjpake_free( &handshake->ecjpake_ctx );
 #endif