spice-widget-egl: Use sizeof instead of hand coded constant

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Snir Sheriber <ssheribe@redhat.com>
2 jobs for master in 5 minutes and 7 seconds
Status Job ID Name Coverage
  Test
passed #437349
fedora

00:05:07

passed #437350
windows

00:03:58