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

Merged-In: I18ed695d6df0e9848e979ad15eb56fba8923726c
Change-Id: I079869f0120fa33893540b72044ddf25c1d04bf2
(cherry picked from commit 69d1a880)