uk_init_tls: Change `uk_thread_tcb_fini` to `uk_thread_uktcb_fini`

uk_init_tls: Change `uk_thread_tcb_fini` to `uk_thread_uktcb_fini`

This commit fixes a typo in which the function`uk_thread_uktcb_fini()`,
as declared in uksched's `tcb_impl.h`, was incorrectly referred to as
`uk_thread_tcb_fini()`.

Signed-off-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #26