build: add missing comma in sources list

This commit adds a missing comma in the sources list. This effects at
least window which produces the following warning where trace_event.h
and src/util.h are concatenated:

Warning: Missing input files:
src\tracing\trace_event.hsrc\util.h

PR-URL: https://github.com/nodejs/node/pull/16613


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>