Add byteswap.h
Add byteswap.h

Trying to make a minimal Alpine Linux image with seasocks and needed this to unbreak the build.
mattgodbolt/seasocks,offa/seasocks,offa/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,offa/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,mattgodbolt/seasocks,offa/seasocks,offa/seasocks,offa/seasocks