imagemagick: add license

pkgs/applications/graphics/ImageMagick/7.0.nix

@@ -84,6 +84,7 @@ stdenv.mkDerivation rec {
     homepage = http://www.imagemagick.org/;
     description = "A software suite to create, edit, compose, or convert bitmap images";
     platforms = platforms.linux ++ platforms.darwin;
+    license = licenses.asl20;
     maintainers = with maintainers; [ the-kenny wkennington ];
   };
 }