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/ipv6.c b/ipv6.c
index a11dd1a..c9b547e 100644
--- a/ipv6.c
+++ b/ipv6.c
@@ -19,7 +19,8 @@
 
 #include <arpa/inet.h>
 
-#include "checksum.h"
+#include "netutils/checksum.h"
+
 #include "config.h"
 #include "debug.h"
 #include "dump.h"