welkin: change license to bsd3

$src/license also see http://simile.mit.edu/welkin/

pkgs/tools/graphics/welkin/default.nix

···
     ];
     hydraPlatforms = [];
     sourceProvenance = with lib.sourceTypes; [ binaryBytecode ];
-    license = lib.licenses.free;
+    license = lib.licenses.bsd3;
     platforms = with lib.platforms; unix;
   };
 }