Export libui_headers as vndk header only library

Emulator EGL uses libui headers and needs to be built
successfully when BOARD_VNDK_VERSION is enabled.

Bug: 64704866
Test: Built aosp_x86 with BOARD_VNDK_VERSION set to current and
      BOARD_VNDK_RUNTIME_DISABLE set to true. Booted on x86 emulator.

Change-Id: Ic26fc14e61330fcc555501c906a64393afe6145d
diff --git a/libs/ui/Android.bp b/libs/ui/Android.bp
index cabe94e..59173cb 100644
--- a/libs/ui/Android.bp
+++ b/libs/ui/Android.bp
@@ -108,4 +108,10 @@
     ],
 }
 
+cc_library_headers {
+    name: "libui_headers",
+    export_include_dirs: ["include"],
+    vendor_available: true,
+}
+
 subdirs = ["tests"]