Makefile.uk: Condition header inclusion on Kconfig

Makefile.uk: Condition header inclusion on Kconfig

Previously musl would add its headers to the include path
unconditionally when its Makefile would be included.
This change conditions this inclusion on whether musl is enabled via
Kconfig.

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