build: disable some warnings

Disabled warnings:

* covered-switch-default: Warns about having a default case when all possible values are covered.
    It's better to report erroneous usage when the switch is improperly used (function called with int instead of enum value).
* switch-enum: Warns about switch not covering all enum values - even when there is a default catch-all.
* used-but-marked-unused: Warns about functions with __attribute__ (unused).
    This attribute means the function is *possibly* unused, not definitely. gobject uses it for priv getters.
    This might be a bogus warning from clang as gcc does not issue such warnings.