Make native_headers product_available

We're fixing the build system to correctly handle the image mutator and
we must explicitly expose native_headers to product modules because some
product modules use the sdk_version property. (Till now it worked
because product modules incorrectly become the core variant)

Bug: 370351758
Test: build
Change-Id: I0a6efb080be1683d44ee33f2cab7d332b7ab1f9c
diff --git a/Android.bp b/Android.bp
index 119c7ea..ba55082 100644
--- a/Android.bp
+++ b/Android.bp
@@ -42,6 +42,7 @@
     export_include_dirs: [
         "include/",
     ],
+    product_available: true,
 }
 
 ndk_headers {