Use more specific includes

We shouldn't be including all of asio, just the parts we need.

Change-Id: I15f5ffa40311644df98fcb9164cac529a423d232
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/example/asio-example.cpp b/example/asio-example.cpp
index 304ceca..ee89a2f 100644
--- a/example/asio-example.cpp
+++ b/example/asio-example.cpp
@@ -1,4 +1,4 @@
-#include <boost/asio.hpp>
+#include <boost/asio/io_context.hpp>
 #include <boost/asio/spawn.hpp>
 #include <sdbusplus/asio/connection.hpp>
 #include <sdbusplus/asio/object_server.hpp>