Makefile.uk: remove malloc_closure.c from ctypes

Makefile.uk: remove malloc_closure.c from ctypes

This change removes malloc_closure.c from the build of the ctypes
extension, as its function is provided by libffi, upon which ctypes
depends.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #15