extensions: Ignore all C warnings in Vala

Vala is being silly again and emitting code that is generating a ton of
C warnings that we can't really do anything about. Just ignore all C
warnings (while keeping the Vala warnings of course).

extensions/goat-exercises/meson.build

@@ -52,6 +52,7 @@ if have_vala and have_gobject_introspection
     ],
     c_args: [
       '-DGETTEXT_PACKAGE="@0@"'.format('org.gimp.extension.goat-exercises'),
+      '-w',
     ],
     install: true,
     install_dir: gimpplugindir /  'extensions' / extension_name,