change MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED to MBEDTLS_KEY_EXCHANGE_PSK_ENABLED

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/include/mbedtls/ssl_ciphersuites.h b/include/mbedtls/ssl_ciphersuites.h
index 48e77d1..d3519f1 100644
--- a/include/mbedtls/ssl_ciphersuites.h
+++ b/include/mbedtls/ssl_ciphersuites.h
@@ -198,7 +198,7 @@
 #endif
 
 /* Key exchanges using a PSK */
-#if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)           || \
+#if defined(MBEDTLS_KEY_EXCHANGE_SOME_NON_PFS_ENABLED)           || \
     defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
 #define MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED
 #endif