use unistd.h instead of getopt.h

getopt.h doesn't exist on AIX and getopt is in unistd.h

src/frontend/mosh-client.cc

@@ -34,7 +34,7 @@
 
 #include <stdlib.h>
 #include <string.h>
-#include <getopt.h>
+#include <unistd.h>
 
 #include "stmclient.h"
 #include "crypto.h"

src/frontend/mosh-server.cc

@@ -51,7 +51,6 @@
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
-#include <getopt.h>
 #include <time.h>
 #include <sys/stat.h>