samples, tests: cleanup void main usage.

Some samples, tests got missed in the switch from void main() to
int main().  Cleanup those samples/tests to use int main().

Signed-off-by: Kumar Gala <kumar.gala@intel.com>

tests/boot/mcuboot_recovery_retention/src/main.c

@@ -10,7 +10,7 @@
 #include <zephyr/kernel.h>
 #include <stdio.h>
 
-void main(void)
+int main(void)
 {
 	printf("Waiting...\n");
 	k_sleep(K_SECONDS(1));
@@ -22,4 +22,6 @@ void main(void)
 	} else {
 		printf("Error, failed to set boot mode: %d\n", rc);
 	}
+
+	return 0;
 }