Add scoutfs_alloc declaration to block.h

block.h has an undeclared use of struct scoutfs_alloc that was relying
on previous headers.  Add a declaraion of the struct (that we don't
dereference) so the header can be used on its own.

Signed-off-by: Zach Brown <zab@versity.com>

kmod/src/block.h

@@ -1,6 +1,8 @@
 #ifndef _SCOUTFS_BLOCK_H_
 #define _SCOUTFS_BLOCK_H_
 
+struct scoutfs_alloc;
+
 struct scoutfs_block_writer {
 	spinlock_t lock;
 	struct list_head dirty_list;