According to information received on #musl: you need to install the kernel headers into the musl prefix
optimally a set of sanitized kernel-headers, like those from http://github.com/sabotage-linux/kernel-headers -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org