Fix std::variant usage

Use the std::get() interface instead of mapbox specific .get() member
interface.

Change-Id: I4ce467c49ea418adba422bbd09fa829acd53da3c
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/version.cpp b/version.cpp
index aa9ee02..69ab415 100644
--- a/version.cpp
+++ b/version.cpp
@@ -115,7 +115,8 @@
     {
         if (p.first == "CurrentPowerState")
         {
-            chassisState = p.second.get<std::string>();
+            chassisState =
+                sdbusplus::message::variant_ns::get<std::string>(p.second);
         }
     }
     if (chassisState.empty())