Include <sys/syscall.h> in ServerBench.cpp for SYS_perf_event_open

`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`.

Resolves weaselab/conflict-set#41.

ServerBench.cpp

@@ -12,6 +12,7 @@
 #include <sys/ioctl.h>
 #include <sys/resource.h>
 #include <sys/socket.h>
+#include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/uio.h>
 #include <thread>