kms: fix build

Fix the following build error:
```
../../modules/video_output/kms.c: In function ‘WindowEnable’:
../../modules/video_output/kms.c:335:9: error: a label can only be part of a statement and a declaration is not a statement
  335 |         struct drm_mode_destroy_dumb destroy_request = {
```

With gcc version 10.2.1 20210110 (Debian 10.2.1-6) and -fsanitize=thread