misc: Prepend srcroot to referenced header paths

[martinjlowm]

Building outside of the project root, requires srcroot to be prepended for paths to resolve to the intended include directories.

This is necessary for something like Nix Linux derivation that sets up a build/ directory and initiates make from it as the working directory.

Reference: https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/kernel/generic.nix#L168