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
Test: BOARD_VNDK_VERSION=current m -j libbcinfo.vendor
Change-Id: I42dad2c0213f2f58886595939a03d800256f9385
diff --git a/Android.bp b/Android.bp
index e048538..c1da62b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -49,6 +49,7 @@
 
 cc_library_headers {
     name: "clang-headers",
+    vendor_available: true,
     host_supported: true,
     generated_headers: clang_generated_headers,
     export_include_dirs: ["include"],