window: Adjust get_image_format() prototype
    
    This adds an unused parameter, but lets us get rid of this new warning
    with gcc 8:
    
    virt-viewer-window.c: In function 'get_image_format':
    virt-viewer-window.c:930:33: warning: cast between incompatible function types from 'GHashTable * (*)(void)' {aka 'struct _GHashTable * (*)(void)'} to 'void * (*)(void *)' [-Wcast-function-type]
         g_once(&image_formats_once, (GThreadFunc)init_image_formats, NULL);
    
    Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
    Acked-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>