Make tinyxml2 build with the VNDK.

When the vndk is enabled (BOARD_VNDK_VERSION=current),
vendor libraries must only link against vendor variants
in the build system. This also means that they receive
restricted sets of headers and that automatic global
headers are restricted.

Test: build tinyxml2 with BOARD_VNDK_VERSION=current
Bug: 37342627
Change-Id: I6938cdda3e718cf1bf97af419eb24b24b4535744