Add zstd to the EFI module list.
    
    cmurf and javierm noticed[0] that we don't have zstd enabled, and that could
    cause issues in some cases for /boot on btrfs subvolumes.  This adds it to our
    module list.
    
    [0] https://github.com/rhinstaller/anaconda/pull/2255#discussion_r359123085
    
    Related: rhbz#1418336
    
    Signed-off-by: Peter Jones <pjones@redhat.com>