use warnings instead of -w

Change-Id: Icce37bf88f36a2da64210cc07e96b8881afec3ab
Reviewed-by: hjk <qthjk@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

doc/fixnavi.pl

@@ -1,5 +1,6 @@
-#! /usr/bin/perl -w
+#! /usr/bin/perl
 
+use warnings;
 use strict;
 
 my @files = ();