Mark libnetutils vendor_available.

libneutils is a library which belongs to vndk-cap. Mark it
vendor_available to enable vndk abi stability checks on it.

Details: https://android-review.googlesource.com/368372

Test: mm -j64

Bug: 38244611

Change-Id: I32938e3e0bcc1e771290d87f561cc06b9b019046
diff --git a/libnetutils/Android.bp b/libnetutils/Android.bp
index 0caf145..9967ef8 100644
--- a/libnetutils/Android.bp
+++ b/libnetutils/Android.bp
@@ -1,5 +1,6 @@
 cc_library_shared {
     name: "libnetutils",
+    vendor_available: true,
 
     srcs: [
         "dhcpclient.c",