btrfs-progs: ioctl: fix compilation if ioctl.h is included standalone

Definition of NULL is missing if ioctl.h is included standalone or when
no other include pulls the definition.

Reported-by: Sergei Trofimovich <slyich@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/ioctl.h b/ioctl.h
index cdfb8882..709e996f 100644
--- a/ioctl.h
+++ b/ioctl.h
@@ -25,6 +25,7 @@ extern "C" {
 
 #include <asm/types.h>
 #include <linux/ioctl.h>
+#include <stddef.h>
 
 #ifndef __user
 #define __user