asio: Explicitly discard return value for spawn

otherwise we get
```
error: ignoring returned value of type ‘boost::asio::deferred_async_operation<...
```

Change-Id: I8929539ee46f9791e69d0ea3558ff60c5d2b5a6d
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/example/asio-example.cpp b/example/asio-example.cpp
index f7f0522..22ae70b 100644
--- a/example/asio-example.cpp
+++ b/example/asio-example.cpp
@@ -373,19 +373,19 @@
 
     // set up a client to make an async call to the server
     // using coroutines (userspace cooperative multitasking)
-    boost::asio::spawn(
+    (void)boost::asio::spawn(
         io,
         [conn](boost::asio::yield_context yield) {
             do_start_async_method_call_one(conn, yield);
         },
         boost::asio::detached);
-    boost::asio::spawn(
+    (void)boost::asio::spawn(
         io,
         [conn](boost::asio::yield_context yield) {
             do_start_async_ipmi_call(conn, yield);
         },
         boost::asio::detached);
-    boost::asio::spawn(
+    (void)boost::asio::spawn(
         io,
         [conn](boost::asio::yield_context yield) {
             do_start_async_to_yield(conn, yield);