xdu: Make format string a string lateral

Clang 4.0.0 shows the warnings below.

```
$ CC=clang make
clang -Wall -Werror -Wextra -pedantic -std=gnu99   -c -o xdu.o xdu.c
xdu.c:218:29: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
            fprintf(stderr, usage);
                            ^~~~~
xdu.c:218:29: note: treat the string as an argument to avoid this
            fprintf(stderr, usage);
                            ^
                            "%s",
xdu.c:226:25: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
        fprintf(stderr, usage);
                        ^~~~~
xdu.c:226:25: note: treat the string as an argument to avoid this
        fprintf(stderr, usage);
                        ^
                        "%s",
```

xdu.c

@@ -215,15 +215,15 @@ char **argv;
     xsetup(&argc, argv);
     if (argc == 1) {
         if (isatty(fileno(stdin))) {
-            fprintf(stderr, usage);
+            fprintf(stderr, "%s", usage);
             exit(1);
         } else {
             parse_file("-");
         }
     } else if (argc == 2 && strcmp(argv[1], "-help") != 0) {
         parse_file(argv[1]);
     } else {
-        fprintf(stderr, usage);
+        fprintf(stderr, "%s", usage);
         exit(1);
     }
     top.size = fix_tree(&top);