lib/posix-process/signal: Add kernel definition of sigset_t

lib/posix-process/signal: Add kernel definition of sigset_t

POSIX specifies that sigset_t can be defined either as integer
type or a structure type. Add kernel definition of sigset_t to
avoid compilation errors caused by flavors of libc that use the
struct definition.

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