clang-headers are marked as vendor_available

The header lib is used by libLLVMBitWriter_3_2 which is a
vendor_available library. (The lib is marked as vendor_available since
it is again used by libbcinfo, which is a vndk-sp library)

Bug: 64425518
Bug: 64395154
Test: BOARD_VNDK_VERSION=current m -j libbcinfo.vendor
Merged-In: I42dad2c0213f2f58886595939a03d800256f9385
Change-Id: I42dad2c0213f2f58886595939a03d800256f9385
(cherry picked from commit 236a7497974d1ee1cf577cd951ce4944e6864819)
1 file changed