I’ve never done this (I got all chirpstack components using THESE fine instructions by mobilefish) but I went and got the source from github, as zip file (click on green “Code” and select “Download ZIP”). Then unzipped, typed “make” (I previously installed golang and make) and it built a binary just fine without any warnings (I’m under ubuntu):
ds-admin@DSE-MFG95:~/ns-src/chirpstack-network-server-master$ dir build/
total 24256
4 ./ 4 ../ 24248 chirpstack-network-server*
So, my question is where did you get the chirpstack ns source?