Config.uk: Add `uksignal` dependency for `LIBMUSL_SIGNAL`

Config.uk: Add `uksignal` dependency for `LIBMUSL_SIGNAL`

`uksignal`  should be selected automatically when `LIBMUSL_SIGNAL` is
enabled, since it relies on underlying syscalls to be implemented by the
kernel.

Signed-off-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #68