Mark libsysutils vendor_available.

libsysutils 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: I3ef1d09c7e836549eb13ce68ac7cdcd5923bde0f