checkpatch: fix perl version 5.12 and earlier incompatibility

A previous patch ("checkpatch: add --types option to report only
specific message types") uses a perl syntax introduced in perl version
5.14.

Use the backward compatible perl syntax instead.
Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -174,9 +174,9 @@ sub hash_save_array_words {
 sub hash_show_words {
 	my ($hashRef, $prefix) = @_;
 
-	if ($quiet == 0 && keys $hashRef) {
+	if ($quiet == 0 && keys %$hashRef) {
 		print "NOTE: $prefix message types:";
-		foreach my $word (sort keys $hashRef) {
+		foreach my $word (sort keys %$hashRef) {
 			print " $word";
 		}
 		print "\n\n";