Fix incorrect command line from avb_add_hash_footer

This change fixes an error that the command line generated from
avb_add_hash_footer contains "a_file --prop_from_file prop_name:a_file",
which is invalid. The file should be an implicit dependency.

Bug: 256148237
Test: N/A
Change-Id: Id0a832bd552d5d79cbc79f250681928140255f91