make libasync_safe vendor_available:true

This, when used with BOARD_VNDK_VERSION flag, enables the
module to be built and installed twice, one for /system,
other for /vendor.

libasync_safe is used by libbacktrace which is in turn used by libs in
/vendor and thus marked as vendor_available. Therefore, libasync_safe
is also marked as vendor_available.

Details: https://android-review.googlesource.com/368372

Bug: 33241851
Test: BOARD_VNDK_VERSION=current m libasync_safe.vendor successful

Change-Id: I4b4de3b99a024d52612109cee3d66b4e5fc12dec
diff --git a/libc/async_safe/Android.bp b/libc/async_safe/Android.bp
index 9e36d97..a835697 100644
--- a/libc/async_safe/Android.bp
+++ b/libc/async_safe/Android.bp
@@ -8,6 +8,7 @@
     ],
 
     name: "libasync_safe",
+    vendor_available: true,
 
     include_dirs: ["bionic/libc"],