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

Merged-In: Ib1490362c5c800e2bdc96ef06e607c93da1006b2
Change-Id: I3ef1d09c7e836549eb13ce68ac7cdcd5923bde0f
(cherry picked from commit a1bdd241)