Move checksum code from clatd to libnetutils.

This allows us to use it elsewhere, for example, in netd.

Bug: 65674744
Test: builds, boots
Test: atest clatd_test clatd_microbenchmark
Change-Id: Ic2cea1cd658ffd23ba9fbe45cb6b28e3b5312005
diff --git a/ipv4.c b/ipv4.c
index fce400f..56d83ac 100644
--- a/ipv4.c
+++ b/ipv4.c
@@ -17,7 +17,8 @@
  */
 #include <string.h>
 
-#include "checksum.h"
+#include "netutils/checksum.h"
+
 #include "debug.h"
 #include "dump.h"
 #include "logging.h"