tools/nolibc: add definitions for standard fds

These are useful for users and will also be used in an upcoming
testcase.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/unistd.h

@@ -13,6 +13,11 @@
 #include "sys.h"
 
 
+#define STDIN_FILENO  0
+#define STDOUT_FILENO 1
+#define STDERR_FILENO 2
+
+
 static __attribute__((unused))
 int msleep(unsigned int msecs)
 {