Change -w to "use warnings;" in tools/get-version

This is more idiomatic and consistent with everywhere else.

tools/get-version

@@ -1,6 +1,7 @@
-#!/usr/bin/perl -w
+#!/usr/bin/perl
 
 use strict;
+use warnings;
 
 open(CONF, "configure.ac") or die "Couldn't open configure.ac: $!\n";