boot/zephyr: switch main return type to 'int'

Adapt to Zephyr's change requiring main to return int.

Signed-off-by: Keith Packard <keithp@keithp.com>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 5201415..d8d4cff 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -530,7 +530,7 @@
 }
 #endif
 
-void main(void)
+int main(void)
 {
     struct boot_rsp rsp;
     int rc;