CMake fixes for MSVC

Had to explicitly include chrono in xlivelessness/xlive/xnetqos.cpp since it wasn't dragged in by another header on my system.

Also switched to using the OPUS_X86_PRESUME_AVX2 option instead of manually specifying the flags. Those flags are different on MSVC, so I was getting a warning; only /arch:AVX2 would be needed. And when you only specify the flags, Opus doesn't enable its hand-rolled optimizations for AVX2. Maybe that was intentional, in which case, adding a comment and switching the flags on the CMake compiler ID is likely the right move. I see in the Opus CI that they only enable the highest OPUS_X86_PRESUME_* option, and looking at the CMake files, it seems like that is correct, so I only enabled the AVX2 option and not all of them.