Fix std::variant usage

This cleans up the use of mapbox only .get() variant apis in favor of
the std::get() interface which both implementations provide.

Change-Id: I9ba8301801d68b391fa5195de37b2b0ecec26edf
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/util.cpp b/util.cpp
index b66f908..28f064c 100644
--- a/util.cpp
+++ b/util.cpp
@@ -544,7 +544,7 @@
     }
 
     reply.read(value);
-    return value.get<std::string>();
+    return sdbusplus::message::variant_ns::get<std::string>(value);
 }
 
 } // namespace mac_address