lib/posix-process: Make header definitions conditional to PIDS

lib/posix-process: Make header definitions conditional to PIDS

Make definitions in private process.h conditional to
CONFIG_LIBPOSIX_PROCESS_PIDS to prevent compile errors if that header
is if that header is included by files compiled unconditionally to
CONFIG_LIBPOSIX_PROCESS_PIDS.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Ioan-Teodor Teugea <ioan_teodor.teugea@stud.acs.upb.ro>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1248