lib/posix-process/signal: Fix coding style of sigset.h

lib/posix-process/signal: Fix coding style of sigset.h

Fix checkpatch issues, prefix macro args with underscore,
remove conditional definition to HAVE_LIBC, include header
for __sigset type.

Checkpatch-Ignore: MACRO_ARG_REUSE
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