releasetools: No-op change to use common.AVB_VBMETA_PARTITIONS.

common.AVB_VBMETA_PARTITIONS was recently added (commit
08c190fc894c7dad112d8ee894c054a36be4bfea) for the same purpose.

Test: TreeHugger
Change-Id: I65572d54c22a753fdef80677377fcc9b684ee16f
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index ebaa867..44719ed 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -450,7 +450,7 @@
     if partition not in needed_partitions:
       continue
     assert (partition in common.AVB_PARTITIONS or
-            partition.startswith('vbmeta_')), \
+            partition in common.AVB_VBMETA_PARTITIONS), \
         'Unknown partition: {}'.format(partition)
     assert os.path.exists(path), \
         'Failed to find {} for {}'.format(path, partition)