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_srv.c b/library/ssl_srv.c
index afac3e1..67dad5f 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -1977,7 +1977,7 @@
 #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
 
 #if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
-    defined(MBEDTLS_ECJPAKE_C)
+    defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
 static void ssl_write_supported_point_formats_ext( mbedtls_ssl_context *ssl,
                                                    unsigned char *buf,
                                                    size_t *olen )
@@ -2005,7 +2005,7 @@
 
     *olen = 6;
 }
-#endif /* MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C || MBEDTLS_ECJPAKE_C */
+#endif /* MBEDTLS_ECDH_C || MBEDTLS_ECDSA_C || MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
 
 #if defined(MBEDTLS_SSL_ALPN )
 static void ssl_write_alpn_ext( mbedtls_ssl_context *ssl,
@@ -2292,7 +2292,7 @@
 #endif
 
 #if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
-    defined(MBEDTLS_ECJPAKE_C)
+    defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
     ssl_write_supported_point_formats_ext( ssl, p + 2 + ext_len, &olen );
     ext_len += olen;
 #endif