Add PRODUCT_EXTRA_VNDK_VERSIONS product variables

PRODUCT_EXTRA_VNDK_VERSIONS should be in _product_list_vars
because it's used as a list with probably more than one version
included/set in differemt makefiles.

Bug: 132046980
Test: $ lunch aosp_x86_ab-userdebug
      $ m -j
      $ find $OUT/system/ -name vndk-* # vndk-27 directory is included

Change-Id: Iee9b7b678a23eed781cc761d820db9ff67ae5d34
diff --git a/core/product.mk b/core/product.mk
index e5df5cd..1d13939 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -302,6 +302,9 @@
 # A flag to override PRODUCT_COMPATIBLE_PROPERTY
 _product_single_value_vars += PRODUCT_COMPATIBLE_PROPERTY_OVERRIDE
 
+# List of extra VNDK versions to be included
+_product_list_vars += PRODUCT_EXTRA_VNDK_VERSIONS
+
 # Whether the whitelist of actionable compatible properties should be disabled or not
 _product_single_value_vars += PRODUCT_ACTIONABLE_COMPATIBLE_PROPERTY_DISABLE