utils: Cleanup strcmp() calls

This comes from the following warning:

```
 src/xdp-utils.c:605:12: note: add parentheses around left hand side expression to silence this warning
           !strcmp (controller, "name=systemd:") != 0 ||
           ^
```

The conversion made here is:

 * `!strcmp (controller, "name=systemd:") != 0`
 * If controller is equal to "name=system:", this will become
   `!0 != 0`
 * Expanding `!0` we have `1 != 0`
 * This evaluates to `TRUE`
 * Therefore, the original expression is merely checking if
   the strings are equal.