Fix typo in macro definition

Fix typo in macro definition

Previously we defined the macro `UK_LIBC_SYSCALL`, however this is not
used anywhere in unikraft; `UK_LIBC_SYSCALLS` is most likely what was
meant. This commit fixes the typo.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Rares Miculescu <miculescur@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #45