Squashfs: factor out remaining zlib dependencies into separate wrapper file

Move zlib buffer init/destroy code into separate wrapper file.  Also
make zlib z_stream field a void * removing the need to include zlib.h
for most files.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
diff --git a/fs/squashfs/squashfs.h b/fs/squashfs/squashfs.h
index ba87db6..9c2f76a 100644
--- a/fs/squashfs/squashfs.h
+++ b/fs/squashfs/squashfs.h
@@ -71,6 +71,8 @@
 extern int squashfs_read_inode(struct inode *, long long);
 
 /* zlib_wrapper.c */
+extern void *squashfs_zlib_init(void);
+extern void squashfs_zlib_free(void *);
 extern int squashfs_zlib_uncompress(struct squashfs_sb_info *, void **,
 				struct buffer_head **, int, int, int, int, int);