I had the same question so did a search. The XIAO-ESP32S3-KWS_inferencing.h is created when the model and example programs are downloaded from the Edge Impulse site when the project is called XIAO-ESP32S3-KWS-inferencing.
However, I found it in the zip file at the github site:
You’ll need to d/l the zip file and unzip it. Then look under the directory for:
TinyML-main\XIAO-esp32-S3-Sense\KeyWordSpotting(KWS)\src