sdl: Limit sdl_grab_end in handle_activation to Windows hosts

There are scenarios on Linux with some SDL versions where
handle_activation is continuous invoked with state = SDL_APPINPUTFOCUS
and gain = 0 while we grabbed the input. This causes a ping-pong when we
grab the input after an absolute mouse entered the window.

As this sdl_grab_end was once introduced to work around a Windows-only
issue (0294ffb9c8), limit it to that platform.

CC: Erik Rull <erik.rull@rdsoftware.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
