Fix define for ssl_conf_curves()

This is a security feature, it shouldn't be optional.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 7db5a3c..554a552 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2641,7 +2641,7 @@
          * } ServerECDHParams;
          */
         const mbedtls_ecp_curve_info **curve = NULL;
-#if defined(MBEDTLS_SSL_SET_CURVES)
+#if defined(MBEDTLS_ECP_C)
         const mbedtls_ecp_group_id *gid;
 
         /* Match our preference list against the offered curves */