fix __retain typings

Author: dcodeIO

std/assembly/rt/index.d.ts

@@ -1,7 +1,7 @@
 declare function __alloc(size: usize, id: u32): usize;
 declare function __realloc(ref: usize, size: usize): usize;
 declare function __free(ref: usize): void;
-declare function __retain(ref: usize): void;
+declare function __retain(ref: usize): usize;
 declare function __release(ref: usize): void;
 declare function __collect(): void;
 declare function __typeinfo(id: u32): u32;