[build-selftests] Run make headers before building selftests

[borkmann]

Fixes BPF CI selftest build after 9fc96c7c19df ("selftests: error out if kernel header files are not yet built").

Link: https://lore.kernel.org/lkml/11b94ce7-d7e3-1935-307b-5a0a0f32739f@collabora.com