Fixup mapbox variant references

This removes all dependencies on the mapbox specific variant api. The
code is now compatible with the drop in std::variant api.

Change-Id: Ie64be86ecae341def54f564eb282fb3b5356cc18
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/dbus_monitor.hpp b/include/dbus_monitor.hpp
index 5dcd5ca..44a7a94 100644
--- a/include/dbus_monitor.hpp
+++ b/include/dbus_monitor.hpp
@@ -6,6 +6,7 @@
 #include <boost/container/flat_set.hpp>
 #include <dbus_singleton.hpp>
 #include <sdbusplus/bus/match.hpp>
+#include <sdbusplus/message/types.hpp>
 
 namespace nlohmann
 {
@@ -14,7 +15,7 @@
 {
     static void to_json(json& j, const sdbusplus::message::variant<Args...>& v)
     {
-        mapbox::util::apply_visitor([&](auto&& val) { j = val; }, v);
+        sdbusplus::message::variant_ns::visit([&](auto&& val) { j = val; }, v);
     }
 };
 } // namespace nlohmann