Don't hardcode perl path but use env instead

Also replace the deprecated "-w" argument with "use warnings;", as
otherwise env would invoke a command called "perl -w".

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>
Closes #5552

scripts/cstyle.pl

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -w
+#!/usr/bin/env perl
 #
 # CDDL HEADER START
 #
@@ -51,6 +51,7 @@
 #
 
 require 5.0;
+use warnings;
 use IO::File;
 use Getopt::Std;
 use strict;