Mark libpng as VNDK

This lib is used by some OEMs. So mark it as VNDK so that we can make
the lib available to them, while keeping the lib in the system
partition.

Bug: 63866913
Test: BOARD_VNDK_VERSION=current m -j libpng libpng.vendor
Change-Id: I6bf73db6118747206b6817a55a7f327e3a68f05b
1 file changed