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: I262ddd4176dabc3b10758fb0b4eb09643aa4911c
diff --git a/libnetutils/Android.bp b/libnetutils/Android.bp
index 1d43775..268496f 100644
--- a/libnetutils/Android.bp
+++ b/libnetutils/Android.bp
@@ -6,6 +6,7 @@
     },
 
     srcs: [
+        "checksum.c",
         "dhcpclient.c",
         "dhcpmsg.c",
         "ifc_utils.c",