gnome.generate_gir() should allow silencing warnings

[ptomato]

I'm using gnome.generate_gir() to process this file, warnlib.h and its companion C file, which contains intentional warnings, for testing purposes. Unfortunately gnome.generate_gir() always appends --warn-all to the g-ir-scanner arguments so this results in a long scary list of warnings on the terminal.

Minor issue, but it would be nice if it was possible to disable warnings for gnome.generate_gir().