backup.pl: Fix Bug13799 - addon restore not working

backup.pl: Fix Bug13799 - addon restore not working

- This fixes the existence check for the addon .ipf file from a check of existence
   of a directory to a check of existence of a file.

Suggested-by: Bernhard Bitsch <bbitsch@ipfire.org>
Tested-by: Bernhard Bitsch <bbitsch@ipfire.org>
Fixes: Bug13799
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>