llext: fix `@kconfig{}` pattern

The `@kconfig{}` pattern does not work properly as the first value in
the `@retval` field, leading to:
```
WARNING: zephyr/include/zephyr/llext/llext.h:296: unexpected token in
comment block while parsing the argument of command retval
```

Signed-off-by: Jordan Yates <jordan@embeint.com>