jakt: Fix signature of default example's main (#4522)

And make sure to cast the i32 to a c_int from the example function.

examples/jakt/default.jakt

@@ -2,6 +2,6 @@ function square(num: i32) -> i32 {
     return num * num
 }
 
-function main() {
-    return square(num: 3)
+function main() throws -> c_int {
+    return square(num: 3) as! c_int
 }