Add a libipchecksum that contains the C IP checksum code.

This is useful for test code that wants to statically link the IP
checksum code. This is not currently possible because libnetutils
is vendor_available and making it available for static linking is
discouraged.

Test: m libnetutils libipchecksum
Original change: https://android-review.googlesource.com/c/platform/system/core/+/1359262

Change-Id: I18fc67d096c7b27804db37621c6662f65d4a78c2
Merged-In: Ic7f1864902581b8a5e3626860565b78c168bdb7f
(cherry picked from commit d4c69114a61d1ff6c5f6a900ae7dd869c2b1df68)
diff --git a/libnetutils/Android.bp b/libnetutils/Android.bp
index 268496f..65371fa 100644
--- a/libnetutils/Android.bp
+++ b/libnetutils/Android.bp
@@ -23,6 +23,21 @@
     export_include_dirs: ["include"],
 }
 
+cc_library_static {
+    name: "libipchecksum",
+
+    srcs: [
+        "checksum.c",
+    ],
+
+    cflags: [
+        "-Wall",
+        "-Werror",
+    ],
+
+    export_include_dirs: ["include"],
+}
+
 cc_binary {
     name: "dhcpdbg",