asn1enum.pl: Make compatible with modern perl

This fixes the following error message:
Experimental keys on scalar is now forbidden at ./asn1enum.pl line 25.

Change-Id: I4680627acfd8f3ed73d32324fe0a54b554268f3b

asn1/asn1enum.pl

@@ -22,7 +22,7 @@ foreach my $k (keys %h) {
 		next;
 	}
 	printf("%s ::= INTEGER {\n", $k);
-	foreach my $r (sort { $a <=> $b } keys $h{$k}) {
+	foreach my $r (sort { $a <=> $b } keys %{$h{$k}}) {
 		printf("\t%s(%d),\n", $h{$k}{$r}, $r);
 	}
 	printf("}\n");