clockoverlay: Fix broken string formatting by strftime() on Windows

Like other foobarA variant APIs on Windows, formatted string
by strftime() is ANSI string, not unicode encoded one.
It would be problematic for non-english locale systems.
We should use unicode version API (wcsftime in this case)
whenever it's possible on Windows.

Part-of: <gstreamer/gst-plugins-base!1107>
33 jobs for !1107 with backport-mr-1093-into-1.18-clockoverlay-win32 in 72 minutes and 4 seconds (queued for 4 seconds)
merge request