test-vectors: Add a soft dependency on DRBG used for KEM tests

Depending on the loaded plugins, it's not necessary. So it's not a hard
dependency.

src/libstrongswan/plugins/test_vectors/test_vectors_plugin.c

@@ -161,6 +161,7 @@ METHOD(plugin_t, get_features, int,
 	static plugin_feature_t f[] = {
 		PLUGIN_NOOP,
 			PLUGIN_PROVIDE(CUSTOM, "test-vectors"),
+				PLUGIN_SDEPEND(DRBG, DRBG_CTR_AES256),
 	};
 	*features = f;
 	return countof(f);