Fix compile error on old libubox

Signed-off-by: Jianhui Zhao <jianhuizhao329@gmail.com>

src/common.h

@@ -38,9 +38,10 @@
 #include <libubox/utils.h>
 #include <libubox/blobmsg.h>
 #include <libubox/kvlist.h>
+#include <libubox/avl-cmp.h>
 #include <libubox/ustream.h>
 
 #include "config.h"
 #include "log.h"
 
-#endif
+#endif