Makefile.uk: Condition TCB reserve on Kconfig

Makefile.uk: Condition TCB reserve on Kconfig

Previously musl's Makefile would unconditionally reserve a TCB size,
regardless of whether musl was selected for build in Kconfig.
This change makes this reservation happen only when musl is selected.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #54