shutter: fix runtime error (missing imagemagick)

Fixes this:

$ shutter
ERROR: imagemagick is missing --> aborting!

Due to 73f1f5eb3969743fe459e7cc36cfb766c7f5318a
("imagemagick: split dev output to fix #9604") which was committed in
the same period as shutter was added to nixpkgs.

pkgs/applications/graphics/shutter/default.nix

···
 
     wrapProgram $out/bin/shutter \
       --set PERL5LIB "${stdenv.lib.makePerlPath perlModules}" \
-      --prefix PATH : "${imagemagick}/bin" \
+      --prefix PATH : "${imagemagick.out}/bin" \
       --set GDK_PIXBUF_MODULE_FILE "$GDK_PIXBUF_MODULE_FILE"
   '';