libceph: fix possible CONFIG_BLOCK build problem

This patch:
    15a0d7b libceph: record message data length
did not enclose some bio-specific code inside CONFIG_BLOCK as
it should have.  Fix that.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/include/linux/ceph/messenger.h b/include/linux/ceph/messenger.h
index b832c0c..cdeebae 100644
--- a/include/linux/ceph/messenger.h
+++ b/include/linux/ceph/messenger.h
@@ -271,8 +271,10 @@
 				size_t length, size_t alignment);
 extern void ceph_msg_data_set_pagelist(struct ceph_msg *msg,
 				struct ceph_pagelist *pagelist);
+#ifdef CONFIG_BLOCK
 extern void ceph_msg_data_set_bio(struct ceph_msg *msg, struct bio *bio,
 				size_t length);
+#endif /* CONFIG_BLOCK */
 
 extern struct ceph_msg *ceph_msg_new(int type, int front_len, gfp_t flags,
 				     bool can_fail);