fsutils/examples: Include unistd.h explicitly

to get the prototypes or macros are defined in it

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

nshlib/nsh_dbgcmds.c

@@ -31,6 +31,7 @@
 #include <string.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <unistd.h>
 
 #include "nsh.h"
 #include "nsh_console.h"