Fix define for ssl_conf_curves()

This is a security feature, it shouldn't be optional.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 4bca71c..318ca46 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -535,7 +535,7 @@
     mbedtls_x509_crl *ca_crl;       /*!< trusted CAs CRLs                   */
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
-#if defined(MBEDTLS_SSL_SET_CURVES)
+#if defined(MBEDTLS_ECP_C)
     const mbedtls_ecp_group_id *curve_list; /*!< allowed curves             */
 #endif
 
@@ -1504,7 +1504,7 @@
                                       unsigned int bitlen );
 #endif /* MBEDTLS_DHM_C && MBEDTLS_SSL_CLI_C */
 
-#if defined(MBEDTLS_SSL_SET_CURVES)
+#if defined(MBEDTLS_ECP_C)
 /**
  * \brief          Set the allowed curves in order of preference.
  *                 (Default: all defined curves.)
@@ -1524,7 +1524,7 @@
  *                 terminated by MBEDTLS_ECP_DP_NONE.
  */
 void mbedtls_ssl_conf_curves( mbedtls_ssl_config *conf, const mbedtls_ecp_group_id *curves );
-#endif /* MBEDTLS_SSL_SET_CURVES */
+#endif /* MBEDTLS_ECP_C */
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 /**