arm64: Fix a broken copyright header in gen_vdso_offsets.sh

I was going to copy this but I didn't want to chase around the build
system stuff so I did it a different way.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Link: https://lore.kernel.org/r/20201017002637.503579-1-palmer@dabbelt.com


Signed-off-by: Will Deacon <will@kernel.org>