Add license

The META files don't know about the license.

Author: sebastic

Makefile.PL

@@ -10,6 +10,7 @@ WriteMakefile(
   NAME => $package_name,
   VERSION_FROM => 'Filter.pm',
   AUTHOR => 'PerlDL Developers <[email protected]>',
+  LICENSE => 'perl',
   CONFIGURE_REQUIRES => {
     'ExtUtils::MakeMaker' => 0,
     'PDL' => '2.096',