std::variant: Fixup .get() usage

This is just a refactor from the mapbox specific .get() interface to the
common std::variant ::get<>() interface.

Tested:
    Built and run through unit tests.

Change-Id: Ic0737f632e80c9dd5b73717ec33d31f245f107db
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/mslverify/util.hpp b/mslverify/util.hpp
index e0d07ca..eefc1cd 100644
--- a/mslverify/util.hpp
+++ b/mslverify/util.hpp
@@ -125,7 +125,7 @@
                    "Get"s, interface, property);
     ::sdbusplus::message::variant<Property> value;
     msg.read(value);
-    return value.template get<Property>();
+    return ::sdbusplus::message::variant_ns::get<Property>(value);
 }
 
 /** @brief Get a property without mapper lookup. */