zephyr: Change main function to return an int.

Zephyr v3.4.0 changed the declaration of the main function to return an
int to allow building Zephyr without the -ffreestanding compiler flag.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>

ports/zephyr/src/zephyr_start.c

@@ -29,11 +29,13 @@
 int real_main(void);
 int mp_console_init(void);
 
-void main(void) {
+int main(void) {
     #ifdef CONFIG_CONSOLE_SUBSYS
     mp_console_init();
     #else
     zephyr_getchar_init();
     #endif
     real_main();
+
+    return 0;
 }