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

@@ -29,7 +29,7 @@
 
 int real_main(void);
 
-void main(void) {
+int main(void) {
     #ifdef CONFIG_CONSOLE_SUBSYS
     console_init();
     #else
@@ -40,4 +40,6 @@ void main(void) {
     // This is needed so the linker includes k_timer_init, z_impl_k_timer_start
     // and z_impl_k_timer_stop, as used by libmicropython.a.
     k_timer_start(NULL, K_MSEC(0), K_MSEC(0));
+
+    return 0;
 }