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/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index dc2ddc1..d9e3e5c 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -41,7 +41,7 @@
 #include "sha512.h"
 #endif
 
-#if defined(MBEDTLS_ECJPAKE_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
 #include "ecjpake.h"
 #endif
 
@@ -172,11 +172,11 @@
 #if defined(MBEDTLS_ECDH_C)
     mbedtls_ecdh_context ecdh_ctx;              /*!<  ECDH key exchange       */
 #endif
-#if defined(MBEDTLS_ECJPAKE_C)
+#if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
     mbedtls_ecjpake_context ecjpake_ctx;        /*!< EC J-PAKE key exchange */
 #endif
 #if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
-    defined(MBEDTLS_ECJPAKE_C)
+    defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
     const mbedtls_ecp_curve_info **curves;      /*!<  Supported elliptic curves */
 #endif
 #if defined(MBEDTLS_KEY_EXCHANGE__SOME__PSK_ENABLED)