examples: Do not include internal headers in ssh_server_fork

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

examples/ssh_server_fork.c

@@ -14,9 +14,9 @@ The goal is to show the API in action.
 #include "config.h"
 
 #include <libssh/callbacks.h>
-#include <libssh/poll.h>
 #include <libssh/server.h>
 
+#include <poll.h>
 #ifdef HAVE_ARGP_H
 #include <argp.h>
 #endif