fix IWYU for async.hpp

async.hpp includes bunch of async header files which is being ignored by
clang-tidy due to IWYU. Hence, use IWYU pragma as per [1].

[1]: https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md#iwyu-pragma-begin_exportsend_exports

Change-Id: I19cd7153b9f7ed3c6ad0cdd73addc701469af191
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/include/sdbusplus/async.hpp b/include/sdbusplus/async.hpp
index 31b3a99..7573cd1 100644
--- a/include/sdbusplus/async.hpp
+++ b/include/sdbusplus/async.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+// IWYU pragma: begin_exports
 #include <sdbusplus/async/context.hpp>
 #include <sdbusplus/async/execution.hpp>
 #include <sdbusplus/async/fdio.hpp>
@@ -7,3 +8,4 @@
 #include <sdbusplus/async/proxy.hpp>
 #include <sdbusplus/async/task.hpp>
 #include <sdbusplus/async/timer.hpp>
+// IWYU pragma: end_exports