little_endian.h uses __always_inline which is defined in stddef.h

diff --git a/include/uapi/linux/byteorder/little_endian.h b/include/uapi/linux/byteorder/little_endian.h
index 601c904..cd98982 100644
--- a/include/uapi/linux/byteorder/little_endian.h
+++ b/include/uapi/linux/byteorder/little_endian.h
@@ -9,6 +9,7 @@
 #define __LITTLE_ENDIAN_BITFIELD
 #endif
 
+#include <linux/stddef.h>
 #include <linux/types.h>
 #include <linux/swab.h>