ui/sdl2: Grab Alt+F4 also under Windows

ui/sdl2: Grab Alt+F4 also under Windows

SDL doesn't grab Alt+F4 under Windows by default. Pressing Alt+F4 thus closes
the VM immediately without confirmation, possibly leading to data loss. Fix
this by always grabbing Alt+F4 on Windows hosts, too.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20230417192139.43263-3-shentey@gmail.com>
(cherry picked from commit 083db9db44c89d7ea7f81844302194d708bcff2b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>