`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.
