fix observed warnings when compiling scsi2sd-util Xcodeproj

[aperezbios]

While compiling SCSI2SD-util with "xcodebuild -workspace SCSI2SD-util/SCSI2SD-util.xcworkspace -scheme SCSI2SD-util -configuration Release", during the CompileXIB stage, the following warnings were emitted:

NSFileCoordinator is doing nothing.
[NSComboBoxCell removeAllItems] should not be called when usesDataSource is set to YES
[NSComboBoxCell addItemsWithObjectValues:] should not be called when usesDataSource is set to YES
[NSComboBoxCell objectValues] should not be called when usesDataSource is set to YES