Clatd changes for move of libipchecksum.

libipchecksum is becoming its own library in
frameworks/libs/net and its include files are no longer under
netutils/. Adjust clatd code accordingly.

Also reorder some include files as suggested by clang-format.

Test: m
Test: atest clatd_test
Test: connects to IPv6-only wifi, testipv4.com shows 10/10
Change-Id: I5e4c73065ce828123aef7e8851227f9bdf444ff9
diff --git a/ipv4.c b/ipv4.c
index c052d03..2be02e3 100644
--- a/ipv4.c
+++ b/ipv4.c
@@ -17,8 +17,7 @@
  */
 #include <string.h>
 
-#include "netutils/checksum.h"
-
+#include "checksum.h"
 #include "debug.h"
 #include "dump.h"
 #include "logging.h"