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/example_app/test.c

@@ -34,6 +34,7 @@
 /* C runtime includes */
 #include <stdio.h>
 #include <stdarg.h>
+#include <stdlib.h>
 #include <time.h>
 #include <string.h>