`ServerBench.cpp` calls `syscall(SYS_perf_event_open, ...)` but did
not include `<sys/syscall.h>`, relying on `<unistd.h>` to transitively
provide the `SYS_*` constants. On toolchains where that does not happen,
the build fails with `SYS_perf_event_open` not declared.
Add the missing include so `server_bench` compiles on any platform that
provides `perf_event_open`.
Resolvesweaselab/conflict-set#41.
ConflictSet_check, ConflictSet_addWrites, ConflictSet_setOldestVersion,
and ConflictSet_destroy return void in C, but the Python wrapper left
their ctypes restype at the default c_int. Set restype = None for each
to match the C API contract and avoid undefined behavior from reading
the return register of void functions.