You enter a nix-shell with the package which includes that header. Locate the package using nix-locate from nix-index.
If you’re building a project that is already packaged in Nixpkgs, you can also use nix-shell '<nixpkgs>' -A yourPackageHere to get a shell with all required dependencies.
Perhaps the OP has found an answer but I’d like to post a solution for those who came later.
with the package linuxHeaders, the headers are at "${linuxHeaders}/include". You may add the path to CPATH/C_INCLUDE_PATH/CFLAGS as you would manually do it with your compiler by cc -I"${linuxHeaders}/include}"