Change case of license in Build.PL

This is to prevent the "License 'GPL_2' is invalid" error when
trying to run the Build.PL script.

Build.PL

@@ -24,7 +24,7 @@ my $file = File::Spec->catfile( "lib", "Starlink", "AST.pm" );
 my $build = Module::Build->new
   (
    module_name => "Starlink::AST",
-   license => "GPL_2",
+   license => "gpl_2",
    author => [
               'Tim Jenness <[email protected]>',
              ],