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);
diff --git a/include/sdbusplus/asio/object_server.hpp b/include/sdbusplus/asio/object_server.hpp
index cf8b7ef..81df2f5 100644
--- a/include/sdbusplus/asio/object_server.hpp
+++ b/include/sdbusplus/asio/object_server.hpp
@@ -235,8 +235,8 @@
         message_t b{m};
 
         // spawn off a new coroutine to handle the method call
-        boost::asio::spawn(io_, std::bind_front(&self_t::after_spawn, this, b),
-                           {});
+        (void)boost::asio::spawn(
+            io_, std::bind_front(&self_t::after_spawn, this, b), {});
 
         return 1;
     }