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