message: Add call_async method

This makes it possible to perform an async method call that is agnostic
to the event loop running the sd_bus state machine.

Change-Id: I32bc0fdf89c44cc6bab1c4622b143d6e06098659
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/meson.build b/test/meson.build
index ac0516e..61eb737 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -24,6 +24,7 @@
     'bus/match',
     'exception/sdbus_error',
     'message/append',
+    'message/call',
     'message/read',
     'message/native_types',
     'message/types',