Reminder to change `rounding_mode` enum definition to move it under `_V1` namespace

[AlexeySachkov]

We have a misplaced definition in vector.hpp which doesn't follow our guidelines around ABI handling:

llvm/sycl/include/sycl/vector.hpp

Lines 54 to 55 in 2e7c2ef

	// TODO: Fix in the next ABI breaking windows.
	enum class rounding_mode { automatic = 0, rte = 1, rtz = 2, rtp = 3, rtn = 4 };

This issue is submitted as a reminder for us to clean this up in the next ABI-breaking window