Fixup std::variant usage

This change refactors mapbox::variant specific usage onto an API that
mapbox::variant and std::variant both support.

Tested:
    Run through unit tests.

Change-Id: Ic35953b4772398495129830ed0d4455409f844ea
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/policy_find.cpp b/policy_find.cpp
index 5771100..51576eb 100644
--- a/policy_find.cpp
+++ b/policy_find.cpp
@@ -44,7 +44,7 @@
 
     if (prop != properties.end())
     {
-        return prop->second.template get<T>();
+        return sdbusplus::message::variant_ns::get<T>(prop->second);
     }
 
     return {};