fix missing standard includes

These were a problem only if arch.h does not include them.

Signed-off-by: Simon Goldschmidt <goldsimon@gmx.de>

contrib/examples/ppp/pppos_example.c

@@ -41,6 +41,8 @@
 
 #include "pppos_example.h"
 
+#include <stdio.h>
+
 #if PPPOS_SUPPORT
 static sio_fd_t ppp_sio;
 static ppp_pcb *ppp;