include/uapi/linux/btrfs.h: define NULL

Copied the NULL definition from include/linux/stddef.h which does not have
it in userspace.

Fixes userspace compilation error:

error: ‘NULL’ undeclared (first use in this function)
    return NULL;

Signed-off-by: Mikko Rapeli <redacted>
---
 include/uapi/linux/btrfs.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/uapi/linux/btrfs.h b/include/uapi/linux/btrfs.h
index b6dec05..a8ae69b 100644
--- a/include/uapi/linux/btrfs.h
+++ b/include/uapi/linux/btrfs.h

@@ -20,6 +20,12 @@
 #define _UAPI_LINUX_BTRFS_H
 #include <linux/types.h>
 #include <linux/ioctl.h>
+#include <linux/stddef.h>
+
+/* for userspace where linux/stddef.h doesn't define this */
+#ifndef NULL
+#define NULL ((void *)0)
+#endif
 
 #define BTRFS_IOCTL_MAGIC 0x94
 #define BTRFS_VOL_NAME_MAX 255

-- 
2.5.0