bcachefs: fix missing include

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.h

@@ -5,6 +5,8 @@
 #include "bcachefs.h"
 #include "alloc_types.h"
 
+#include <linux/hash.h>
+
 struct bkey;
 struct bch_dev;
 struct bch_fs;