zephyr: fix include in hello-world app

Update include due to changes in Zephyr. This is related to commit
f1d189c48649f179e6a9fcd71abe90cff82ad6b5.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/samples/zephyr/hello-world/src/main.c b/samples/zephyr/hello-world/src/main.c
index 189b1fb..027bb5f 100644
--- a/samples/zephyr/hello-world/src/main.c
+++ b/samples/zephyr/hello-world/src/main.c
@@ -5,7 +5,7 @@
  */
 
 #include <zephyr.h>
-#include <misc/printk.h>
+#include <sys/printk.h>
 
 void main(void)
 {