Makefile.uk: Fix build-time infinite loop

Makefile.uk: Fix build-time infinite loop

This commit removes the dependency on the $(UK_CONFIG_OUT) file, which
is not necessary for `lib-click` to build and actually
causes an infinite loop in the build process.

Signed-off-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #6