CI: Enable introspection again for the Windows build

The underlying bug was fixed in glib and backported in MSYS2:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1665

.gitlab-ci/test-msys2.sh

@@ -60,14 +60,12 @@ pkg-config --modversion pango
 ccache --zero-stats
 ccache --show-stats
 export CCACHE_DISABLE=true
-# FIXME: introspection disabled for now because of
-# https://gitlab.gnome.org/GNOME/gobject-introspection/-/issues/340
 meson \
     -Dx11-backend=false \
     -Dwayland-backend=false \
     -Dwin32-backend=true \
     -Dvulkan=disabled \
-    -Dintrospection=disabled \
+    -Dintrospection=enabled \
     -Dgtk:werror=true \
     _build
 unset CCACHE_DISABLE