Move _XOPEN_SOURCE define to library CFLAGS

Move _XOPEN_SOURCE define to library CFLAGS

Putting it into CFLAGS changes the behavior for all sources files in a
Unikraft build and changes them from the implicit _DEFAULT_SOURCE to
_XOPEN_SOURCE, which can break applications/libraries.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #33