GWin32VolumeMonitor: Sort the volumes correctly

Use a static GQueue to form the GList of mounts by appending (which
is fast, because GQueue tracks the tail pointer of its internal GList),
then return that GList. This way we don't need to form the list
by prepending, which would have made it necessary to reverse it before
returning.

If the list is not ordered correctly, local drives in GTK places sidebar
are shown in reverse order.
7 jobs for w32-mount-sort in 13 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #214398
cross-android_api21_arm64

00:05:41

passed #214399
cross-android_api28_arm64

00:05:42

passed #214400
cross-mingw64

00:05:52

passed #214397
fedora-x86_64

00:08:18

passed #214401
win32
msys2-mingw32

00:08:41

passed #214402
win32
vs2017-x64

00:08:02

 
  Coverage
passed #214403
coverage

00:04:41

74.5%