ui/cocoa: Suppress NSFileHandlingPanelOKButton deprecation warning

OSX 10.13 deprecates the NSFileHandlingPanelOKButton constant, and
would rather you use NSModalResponseOK, which was introduced in OS 10.9.
Use the recommended new constant name, with a backward compatibility
define if we're building on an older OSX.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 20180529181523.19185-1-peter.maydell@linaro.org