Add vendor_available:true to libpng

Bug: 38244611
Test: build sailfish
Change-Id: If514202184e125742a3205052f06a84d2195525d
diff --git a/Android.bp b/Android.bp
index 65bb9ba..93e07e3 100644
--- a/Android.bp
+++ b/Android.bp
@@ -79,6 +79,7 @@
 
 cc_library {
     name: "libpng",
+    vendor_available: true,
     host_supported: true,
     defaults: ["libpng-defaults"],
     target: {