interface: add signal support

Change-Id: Ice572b6794e7cb2426a2e86b7bb92ca3d0ac7ae1
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/sdbusplus/message.hpp b/sdbusplus/message.hpp
index 74e0cdd..053cd61 100644
--- a/sdbusplus/message.hpp
+++ b/sdbusplus/message.hpp
@@ -137,6 +137,9 @@
         sd_bus_send(b, this->get(), nullptr);
     }
 
+    /** @brief Perform a 'signal-send' call. */
+    void signal_send() { method_return(); }
+
     friend struct sdbusplus::bus::bus;
 
     private: