Turn off clang-format for Linkdef files

### Explain what you would like to see improved and how.

Linkdef files should not be subject to clang-format checks. They force us to write lines such as

```
#pragma link C++ class Event + ;
```

(Note the blanks around the `+`)

### ROOT version

master

### Installation method

n/a

### Operating system

n/a

### Additional context

_No response_