use 'ne' for string comparison

Change-Id: I0d059933786119c3c26202c7b48d793404e5a0eb
diff --git a/postprocess/signing/signing.pl b/postprocess/signing/signing.pl
index fc17092..875076a 100644
--- a/postprocess/signing/signing.pl
+++ b/postprocess/signing/signing.pl
@@ -47,7 +47,7 @@ if ( $#ARGV < 2 ) {
    exit(1);
}
@args = parse_options();
get_exclude_files() if ($opt_exclude != "");
get_exclude_files() if ($opt_exclude ne "");
@files_to_sign = get_files(\@args);
if ( $opt_log ) {               # logging
    open(LOG,">$opt_log") || die "Can't open log file $opt_log\n";