add necessary hello.c code as we dont have linking step

examples/hello.c

@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+int main(void) {
+    printf("add(2, 2) = %d\n", add(2, 2));
+}

examples/hello.ry

@@ -47,7 +47,7 @@ fn func_refer_param(b: i32) i32 {
 }
 
 fn add(a: i32, b: i32) i32 {
-    return 69 + 69;
+    return a + b;
 }
 
 fn and_fn() bool {