Some instances of UInitOnce are missing initialization, have no " = U_INITONCE_INITIALIZER".
For static/global instances, most of them, this is probably not causing an actual problem, but it is cleaner to have it. Non-static instances should really have it.
Adding it across the board, for consistency.
This may be revisited yet again, adding C++11 style init to UInitOnce itself. But that is a bigger change - most uses have the initializer already.