MAINTAINERS: Add include/linux/tnum.h to BPF CORE

Maintainers of the kerne/bpf/tnum.c are also the maintainers of the
corresponding header file include/linux/tnum.h.

Add the file entry for include/linux/tnum.h to the appropriate section
in MAINTAINERS.

Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220831034039.17998-1-shung-hsi.yu@suse.com