@griffinboy yes it's included within the RTNeural code folder, but it's only available in the translation unit that compiles the RTNeural code (the hi_tools module).
In order to make it accessible on other translation units and your own nodes you have to add this line to hi_tools/hi_tools.h:
#include "hi_neural/RTNeural/modules/xsimd/xsimd.hpp"
The place doesn't really matter, but somewhere around line 150 would be a good spot.