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/clatd_test.cpp b/clatd_test.cpp
index 7705a36..8e49824 100644
--- a/clatd_test.cpp
+++ b/clatd_test.cpp
@@ -29,10 +29,10 @@
 #include "tun_interface.h"
 
 extern "C" {
-#include "checksum.h"
 #include "clatd.h"
 #include "config.h"
 #include "getaddr.h"
+#include "netutils/checksum.h"
 #include "translate.h"
 #include "tun.h"
 }