meson_options.txt: don't use str when defining bool default values

meson_options.txt: don't use str when defining bool default values

Avoids this deprecation notice from `meson` 1.3.2:

```
:: NOTICE: Future-deprecated features used:
::  * 1.1.0: {'"boolean option" keyword argument "value" of type str'}
```

Other options already use this syntax so backward compat should remain unchanged.

Signed-off-by: Simon Deziel <simon.deziel@canonical.com>