ssl_tls: Enable Suite B with subset of ECP curves

Make sure the code compiles even if some curves are not defined.

Fixes #1591
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 38690fa..424b071 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -9043,8 +9043,12 @@
 
 #if defined(MBEDTLS_ECP_C)
 static mbedtls_ecp_group_id ssl_preset_suiteb_curves[] = {
+#if defined(MBEDTLS_ECP_DP_SECP256R1_ENABLED)
     MBEDTLS_ECP_DP_SECP256R1,
+#endif
+#if defined(MBEDTLS_ECP_DP_SECP384R1_ENABLED)
     MBEDTLS_ECP_DP_SECP384R1,
+#endif
     MBEDTLS_ECP_DP_NONE
 };
 #endif