Turns out AudioConvertHostTimeToNanos and AudioGetCurrentHostTime are macOS-only APIs, which prevents apps using GStreamer on iOS from being accepted into App Store. This commit replaces those functions with a manual version of what they do - mach_absolute_time() for the current time, and data from mach_timebase_info() at the beginning to convert host timestamps to nanoseconds. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6789>