ui/cocoa: Remove stray tabs

The ui/cocoa.m file has just three lines with hardcoded tabs; fix them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1387886052-27067-1-git-send-email-peter.maydell@linaro.org