Have `make ssl-opt` generate `tls13-compat.sh` (make edition)

This is necessary if you haven't run `make generated_files` first.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/Makefile b/Makefile
index 00de608..e4d98c9 100644
--- a/Makefile
+++ b/Makefile
@@ -30,6 +30,7 @@
 
 ssl-opt: lib mbedtls_test
 	$(MAKE) -C programs ssl-opt
+	$(MAKE) -C tests ssl-opt
 
 lib:
 	$(MAKE) -C library