Make libhidl_gtest_helper vendor_available.

This is needed for vts tests to be marked as vendor tests. Vendor vts
tests are useful when we're testing framework hals.

Bug: 231517902

Test: build vendor VtsHalCameraProviderV2_4TargetTest

Change-Id: I5abab3c29f2212a0680fd179462ff4c254afe1b9
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>
diff --git a/Android.bp b/Android.bp
index c5d129e..bd7c1a8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -48,6 +48,7 @@
 
 cc_library_headers {
     name: "libhidl_gtest_helper",
+    vendor_available: true,
     host_supported: true,
     export_include_dirs: ["gtest_helper"],
 }