[WATCHDOG] includes for sample watchdog program.

Add missing includes to sample watchdog program.
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Signed-off-by: NWim Van Sebroeck <wim@iguana.be>

Documentation/watchdog/src/watchdog-simple.c

+#include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 #include <fcntl.h>
 
 int main(int argc, const char *argv[]) {