linux-bpf-dev: install header to linux/ subdirectory

Ubuntu ships the header as:

/usr/include/x86_64-linux-gnu/linux/vmlinux.h

Debian ships it as:

/usr/include/x86_64-linux-gnu/linux/bpf/vmlinux.h

This means packages need to ifdef the include depending on the vendor, which is not great. It hasn't shipped in Debian stable yet, so fix it to match Ubuntu's, which is in multiple stable releases already.

https://packages.ubuntu.com/oracular/amd64/linux-bpf-dev/filelist

https://packages.debian.org/sid/amd64/linux-bpf-dev/filelist

In Ubuntu this has shipped in stable releases including Noble 24.04 LTS so it's much harder to fix it the other way around:

https://packages.ubuntu.com/search?keywords=linux-bpf-dev&searchon=names&suite=all&section=all