kconfigcheck.toml: add Librem 5 category

We've been disabling CONFIG_RS9116_FLASH_MODE for years at this point,
so this doesn't do anything but enforce the status quo.

Rationale for this can be found in the commit
ddf56b51.

This is deliberately not added to the community check as it is
device-specific.

Closes https://gitlab.com/postmarketOS/pmaports/-/issues/3157