Declare the headers of the ndk_library

This creates a dependency edge between the ndk_library and its headers,
which should be a no-op in regular Soong builds. This dependency edge
will be used in the Multi-tree project to export the relevant .h files into a well
known location

Test: m nothing
Bug: 239044713
Change-Id: I2d21074b79c201911fb0cea0a521a5623e7f17b3
diff --git a/libs/nativewindow/Android.bp b/libs/nativewindow/Android.bp
index dd07319..d7db6bd 100644
--- a/libs/nativewindow/Android.bp
+++ b/libs/nativewindow/Android.bp
@@ -61,6 +61,9 @@
 
     // Android O
     first_version: "26",
+    export_header_libs: [
+        "libnativewindow_ndk_headers",
+    ],
 }
 
 cc_library {