Include stddef to build on trusty (for travis)

Signed-off-by: Stefano Babic <sbabic@denx.de>

src/uboot_env.c

@@ -15,6 +15,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdint.h>
+#include <stddef.h>
 #include <dirent.h>
 #include <unistd.h>
 #include <string.h>