Enable -Werror for all of qtsvg

Change-Id: Ica038c81648e3d3c7c2418aacf40eef1f62602b6
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: Iebfffe157538b0a0573e9bfbed7eeac6f53390b1