bpf: offload: add a license header

I forgot to add a license on kernel/bpf/offload.c.  Luckily I'm
still the only author so make it explicitly GPLv2.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>