Need to declare variables volatile when using gcc 8.1.0

[elekven-m]

In ideviceinstaller.c, around lines 110 to 117, the variables wait_for_command_complete, use_notifier, notification_expected, is_device_connected, command_completed, err_occurred, and notified need to be declared volatile when compiling with gcc 8.1.0 with standard make or else they are not re-read in the spinloops which is required as they are used to pass info from the background thread to the foreground (which is spinning).