Btrfs: compat code fixes

The btrfs git kernel trees is used to build a standalone tree for
compiling against older kernels.  This commit makes the standalone tree
work with 2.6.27

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/super.c

@@ -37,6 +37,8 @@
 #include <linux/ctype.h>
 #include <linux/namei.h>
 #include <linux/miscdevice.h>
+#include <linux/version.h>
+#include "compat.h"
 #include "ctree.h"
 #include "disk-io.h"
 #include "transaction.h"