gs-update-dialog.ui: Add left and right margin to os_update_box

The current os_update_box looks a bit weird, because the left and right
margin is missing.

I've added a 18px left&right margin. Now it looks better.

#317
1 job for master in 6 minutes and 13 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #11144
build-gnome-software

00:06:13