Mark libdiskconfig vendor_available.

libdisckonfig 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: I079869f0120fa33893540b72044ddf25c1d04bf2