Fix "unhandled metric 11" warning in QSvgGenerator::metric().

cea58f4b77e1639e5671cf424544d4948fb8e9ef introduced a new enum value
that wasn't handled by the switch statement.

Change-Id: I40a44cb7694f9a0f454bfadf014d09c67538daad
Task-number: QTBUG-43197
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>