bcachefs: Remove duplicated include in backpointers.c

The header files bbpos.h is included twice in backpointers.c,
so one inclusion of each can be removed.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=10783
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Yang Li

fs/bcachefs/backpointers.c

@@ -3,7 +3,6 @@
 #include "bbpos.h"
 #include "alloc_background.h"
 #include "backpointers.h"
-#include "bbpos.h"
 #include "bkey_buf.h"
 #include "btree_cache.h"
 #include "btree_update.h"