Makefile.uk: Add sources for POSIX_USER functions

Makefile.uk: Add sources for POSIX_USER functions

Since libc style syscall stubs are disabled when using musl, many of the
sources should be always included to avoid `undefined refrence` errors.

Signed-off-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@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: #20