btrfs: make btrfs_compressed_bioset static

The 'btrfs_compressed_bioset' struct isn't exported outside of the
fs/btrfs/compression.c file, so make it static to fix the following
sparse warning:

fs/btrfs/compression.c:40:16: warning: symbol 'btrfs_compressed_bioset' was not declared. Should it be static?

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>