doc/texi2pod: add "use warnings" directive

The script was previously run with perl -w through the shebang
command. Now that the script is executed through direct perl invocation
the -w in the shebang command is ignored. This patch re-enables "use
warnings" whatever way the script is invoked.
Idea-By: jamal <jamrial@gmail.com>

doc/texi2pod.pl

-#! /usr/bin/perl -w
+#! /usr/bin/perl
 
 #   Copyright (C) 1999, 2000, 2001 Free Software Foundation, Inc.
 
@@ -23,6 +23,8 @@
 # markup to Perl POD format.  It's intended to be used to extract
 # something suitable for a manpage from a Texinfo document.
 
+use warnings;
+
 $output = 0;
 $skipping = 0;
 %sects = ();