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: I374b1529456c4c71ac419b4684f2fd215c68e791
diff --git a/libsync/Android.bp b/libsync/Android.bp
index 99c88cf..b6b4a6e 100644
--- a/libsync/Android.bp
+++ b/libsync/Android.bp
@@ -27,6 +27,9 @@
     name: "libsync",
     symbol_file: "libsync.map.txt",
     first_version: "26",
+    export_header_libs: [
+        "libsync_headers",
+    ],
 }
 
 cc_defaults {