libical: don't build examples

One of these fails to link in a static build. I've fixed it
upstream, but I don't think there's any value building the examples
in Nixpkgs regardless, so let's disable them.

Link: https://github.com/libical/libical/pull/932

pkgs/by-name/li/libical/package.nix

···
   cmakeFlags =
     [
       "-DENABLE_GTK_DOC=False"
+      "-DLIBICAL_BUILD_EXAMPLES=False"
       "-DGOBJECT_INTROSPECTION=${if withIntrospection then "True" else "False"}"
       "-DICAL_GLIB_VAPI=${if withIntrospection then "True" else "False"}"
     ]