GTK: Fix builds against latest stable GLib with MSVC

GLib-2.68.x now considers warning C4098 ('void' function returning a value) as
an error, so avoid doing that.
12 jobs for latest.glib.msvc in 21 minutes and 41 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  .Pre
passed #1255959
style-check-diff

00:08:32

 
  Build
passed #1255960
fedora-x86_64

00:21:03

passed #1255962
installed-tests

00:19:42

passed #1255964
macos
macos

00:11:44

passed #1255963
win32-ps
msys2-mingw64

00:06:09

passed #1255961
release-build

00:04:06

 
  Analysis
failed #1255966
asan allowed to fail
asan-build

00:10:35

passed #1255965
static-scan

00:05:57

 
  Docs
passed #1255967
reference

00:21:41

 
  Flatpak
manual #1255968
flatpak allowed to fail manual
flatpak-manual:demo
manual #1255970
flatpak allowed to fail manual
flatpak-manual:icon-browser
manual #1255969
flatpak allowed to fail manual
flatpak-manual:widget-factory
 
Name Stage Failure
failed
asan-build Analysis
 - css: 214 total, 214 pass, 0 fail
Processing gtk suite a11y:
- a11y: 22 total, 22 pass, 0 fail
Processing gtk suite tools:
- tools: 4 total, 3 pass, 1 fail
Processing gtk suite reftest:
- reftest: 132 total, 131 pass, 1 fail
Cleaning up file based variables
ERROR: Job failed: exit code 1