Make slang headers available for vendor use

Bug: 131709055
Test: manual
Change-Id: I7eb4eb74b365d5ef289e88bc1d8d8b19457eea7f
(cherry picked from commit a0de4c592c8aee54421f0b011b22d6cc5effa0d4)
diff --git a/Android.bp b/Android.bp
index 8a394fa..c8fe151 100644
--- a/Android.bp
+++ b/Android.bp
@@ -69,6 +69,7 @@
 // Exported header files
 cc_library_headers {
     name: "slang_headers",
+    vendor_available: true,
     export_include_dirs: ["."],
     host_supported: true,
     target: {