test: inject unused variable to verify -Werror catches it

Author: abhinavagarwal07

sshfs.c

@@ -4240,6 +4240,7 @@ int main(int argc, char *argv[], __unused char *envp[], char **exec_path)
 int main(int argc, char *argv[])
 #endif
 {
+	int intentional_unused_warning_var;
 	int res;
 	struct fuse_args args = FUSE_ARGS_INIT(argc, argv);
 	char *tmp;