Mark libxml2 as vendor_available

The lib is being depended by some of vendor modules such as
android.hardware.power@1.1-service.*.

Bug: 37560614
Test: BOARD_VNDK_VERSION=current m -j libxml2.vendor
Merged-In: I1bec500ea79edf6b7e4d6fa46bbd7cd53b299d65
Change-Id: I1bec500ea79edf6b7e4d6fa46bbd7cd53b299d65
(cherry picked from commit a3690fa4d8a93afd4c98b74b5a690b7e91b16b26)
diff --git a/Android.bp b/Android.bp
index dcbd464..01fc87b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -33,6 +33,7 @@
 // We deliberately exclude nanoftp.c and nanohttp.c, the trio library, and zlib.
 cc_library {
     name: "libxml2",
+    vendor_available: true,
     host_supported: true,
     srcs: [
         "SAX.c",