Cocoa: Silence tablet event warning on 10.6

10.6 sends tablet events for trackpad interaction,
but not proximity events. The reason for this is not
known. The events are discard.

Silence the warning to avoid flooding the console.

Change-Id: I6c215e8c15fc5b4b80e68b00f172121fc4d251b7
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>