Mark an SP-NDK, libsync.so as VNDK libs.

libsync.so is categorized as SP-NDK and vendor_available.
However, it is marked as VNDK because it is needed by some VNDK libs.

Bug: 63866913
Test: build and boot with BOARD_VNDK_VERSION=current
Change-Id: I116a442ce284a6e5ba1c76f99d44329a9f00fed7