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>

fs/btrfs/compression.c

@@ -37,7 +37,7 @@
 #include "file-item.h"
 #include "super.h"
 
-struct bio_set btrfs_compressed_bioset;
+static struct bio_set btrfs_compressed_bioset;
 
 static const char* const btrfs_compress_types[] = { "", "zlib", "lzo", "zstd" };