MAINTAINERS: add myself to co-maintain ZSTD

Recently the ZSTD tree has been removed from linux-next due to lack of
updates for last year [1]. As there are several users of ZSTD in kernel
we need to keep the code maintained and updated. I'll act as a backup to
get the ZSTD upstream to linux, Nick is OK with that [2].

[1] https://lore.kernel.org/all/20250216224200.50b9dd6a@canb.auug.org.au/
[2] https://github.com/facebook/zstd/issues/4262#issuecomment-2691527952

CC: Nick Terrell <terrelln@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Nick Terrell <terrelln@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>