MAINTAINERS: Add btf headers to BPF

BPF folks maintain these and they're not picked up by the current
MAINTAINERS entries.

Files caught by the added globs:

  include/linux/btf.h
  include/linux/btf_ids.h
  include/uapi/linux/btf.h

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210924193557.3081469-1-davemarchevsky@fb.com