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_microbenchmark.c b/clatd_microbenchmark.c
index 83be006..91b0996 100644
--- a/clatd_microbenchmark.c
+++ b/clatd_microbenchmark.c
@@ -41,7 +41,7 @@
 #include <time.h>
 #include <unistd.h>
 
-#include "checksum.h"
+#include "netutils/checksum.h"
 #include "tun.h"
 
 #define DEVICENAME "clat4"