Build libmockdrmcryptoplugin with VNDK.

Bug: 37342627
Test: BOARD_VNDK_VERSION=current m -j64 libmockdrmcryptoplugin
Change-Id: Ided0ce365e0f13b9c4772c99508e253fa627f0be
diff --git a/drm/mediadrm/plugins/mock/Android.bp b/drm/mediadrm/plugins/mock/Android.bp
index 7f44819..abd1884 100644
--- a/drm/mediadrm/plugins/mock/Android.bp
+++ b/drm/mediadrm/plugins/mock/Android.bp
@@ -22,6 +22,8 @@
     vendor: true,
     relative_install_path: "mediadrm",
 
+    header_libs: ["media_plugin_headers"],
+
     shared_libs: [
         "libutils",
         "liblog",