monitor: include unistd.h in main.c

This fixes a build break on some systems, specifically the
raspberry Pi 3 (ARM):

monitor/main.c: In function ‘open_packet’:
monitor/main.c:176:3: error: implicit declaration of function ‘close’; did you mean ‘pclose’? [-Werror=implicit-function-declaration]
  176 |   close(fd);
      |   ^~~~~
      |   pclose

monitor/main.c

@@ -32,6 +32,7 @@
 #include <string.h>
 #include <getopt.h>
 #include <signal.h>
+#include <unistd.h>
 #include <arpa/inet.h>
 #include <sys/socket.h>
 #include <linux/genetlink.h>