btrfs: cleanup kobject.h includes

The kobject should be pulled in via sysfs.h and that needs to include it
because it needs various definitions like kobj_attribute or kobject.

Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/sysfs.h b/fs/btrfs/sysfs.h
index 4243fcf..611e103 100644
--- a/fs/btrfs/sysfs.h
+++ b/fs/btrfs/sysfs.h
@@ -3,6 +3,8 @@
 #ifndef BTRFS_SYSFS_H
 #define BTRFS_SYSFS_H
 
+#include <linux/kobject.h>
+
 enum btrfs_feature_set {
 	FEAT_COMPAT,
 	FEAT_COMPAT_RO,