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/Android.bp b/Android.bp
index 82295f3..e0eeb2e 100644
--- a/Android.bp
+++ b/Android.bp
@@ -21,7 +21,6 @@
 filegroup {
     name: "clatd_common",
     srcs: [
-        "checksum.c",
         "config.c",
         "clatd.c",
         "dns64.c",
@@ -90,7 +89,9 @@
     defaults: ["clatd_defaults"],
     srcs: [
         "clatd_microbenchmark.c",
-        "checksum.c",
         "tun.c",
     ],
+    shared_libs: [
+        "libnetutils",
+    ],
 }