bcache: Add missing #include <linux/prefetch.h>

m68k/allmodconfig:

drivers/md/bcache/bset.c: In function ‘bset_search_tree’:
drivers/md/bcache/bset.c:727: error: implicit declaration of function ‘prefetch’

drivers/md/bcache/btree.c: In function ‘bch_btree_node_get’:
drivers/md/bcache/btree.c:933: error: implicit declaration of function ‘prefetch’

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kent Overstreet <koverstreet@google.com>
diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index 0b33aac..d4f2164 100644
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -10,6 +10,7 @@
 #include "debug.h"
 
 #include <linux/random.h>
+#include <linux/prefetch.h>
 
 /* Keylists */