loop: add blk-mq.h include

Looks like we pull it in through other ways on x86, but we fail
on sparc:

In file included from drivers/block/cryptoloop.c:30:0:
drivers/block/loop.h:63:24: error: field 'tag_set' has incomplete type
struct blk_mq_tag_set tag_set;

Add the include to loop.h, kill it from loop.c.
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/loop.c

@@ -75,7 +75,6 @@
 #include <linux/sysfs.h>
 #include <linux/miscdevice.h>
 #include <linux/falloc.h>
-#include <linux/blk-mq.h>
 #include "loop.h"
 
 #include <asm/uaccess.h>

drivers/block/loop.h

@@ -11,6 +11,7 @@
 
 #include <linux/bio.h>
 #include <linux/blkdev.h>
+#include <linux/blk-mq.h>
 #include <linux/spinlock.h>
 #include <linux/mutex.h>
 #include <linux/workqueue.h>