Fix std::variant usage

This change makes it possible to use std::variant as a drop in
replacement for mapbox::variant.

Change-Id: Ia729c3a7832eec4dd6cddadba9bc160b6ba9ad90
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/test/test.cpp b/test/test.cpp
index 2327239..cf61b84 100644
--- a/test/test.cpp
+++ b/test/test.cpp
@@ -425,7 +425,8 @@
             sig.read(sigInterface);
             assert(sigInterface == "xyz.openbmc_project.Example.Iface1");
             sig.read(sigProperties);
-            assert(sigProperties["ExampleProperty1"] == "changed");
+            assert(sdbusplus::message::variant_ns::get<std::string>(
+                       sigProperties["ExampleProperty1"]) == "changed");
         }
     }
 }