reverted change to MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED, as it appears it could be causing issues

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/include/mbedtls/ssl_ciphersuites.h b/include/mbedtls/ssl_ciphersuites.h
index 80d5c7e..cc9f8d8 100644
--- a/include/mbedtls/ssl_ciphersuites.h
+++ b/include/mbedtls/ssl_ciphersuites.h
@@ -185,7 +185,7 @@
 #endif
 
 /* Key exchanges that don't involve ephemeral keys */
-#if defined(MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)
 #define MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED
 #endif