samples: hello_world: use printf

Instead of printk. This change aligns the sample a bit more to the
canonical "hello world" sample in C. Also, samples should in general be
as portable as possible.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

samples/hello_world/src/main.c

@@ -4,10 +4,10 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr/kernel.h>
+#include <stdio.h>
 
 int main(void)
 {
-	printk("Hello World! %s\n", CONFIG_BOARD);
+	printf("Hello World! %s\n", CONFIG_BOARD);
 	return 0;
 }