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/tests/Makefile b/tests/Makefile
index 405d175..63df02c 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -59,10 +59,14 @@
 GENERATED_FILES = $(GENERATED_DATA_FILES) $(GENERATED_CRYPTO_DATA_FILES)
 GENERATED_FILES += src/test_keys.h src/test_certs.h
 
+# Generated files needed to (fully) run ssl-opt.sh
+.PHONY: ssl-opt
+
 opt-testcases/tls13-compat.sh: scripts/generate_tls13_compat_tests.py
 	echo "  Gen   $@"
 	$(PYTHON) scripts/generate_tls13_compat_tests.py -o $@
 GENERATED_FILES += opt-testcases/tls13-compat.sh
+ssl-opt: opt-testcases/tls13-compat.sh
 
 .PHONY: generated_files
 generated_files: $(GENERATED_FILES)