build: don't hide the stdbool include from g-ir-scanner

[lazka]

g-i recently lost the ability to infer the bool type without a stdbool include.
While this is about to be fixed in g-i (https://gitlab.gnome.org/GNOME/gobject-introspection/merge_requests/116)
I don't see a reason why we should hide this in the first place, so remove the ifdef guard.

[ebassi]

The reason why I ignored it was because older g-ir-scanner versions would barf on bool.

I guess this means that graphene needs to depend on a minimum version of gobject-introspection.

[lazka]

OK, CI uses Debian stretch and I don't see any errors/warnings there. So >= 1.50 should work for example.

[ebassi]

Okay, let's merge this as is; I can add a requirement on 1.50 (and bump up the requirement for GLib 2.50) if that breaks elsewhere.

[lazka]

Thanks!