tools: undump: rely on BTF instead of header files
tools: undump: rely on BTF instead of header files

BTF, provided by most distributions nowadays, is a more robust way to
provide kernel structure than the inclusion of kernel headers. Rely on
BTF by default and only fall back on kernel headers if it's
unavailable.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace