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/test/test_callout.cpp b/test/test_callout.cpp
index 3ac2c3e..d5fec71 100644
--- a/test/test_callout.cpp
+++ b/test/test_callout.cpp
@@ -74,14 +74,20 @@
         ASSERT_EQ(callout->ts(), ts);
         ASSERT_EQ(callout->path(), calloutPath);
         ASSERT_EQ(callout->buildDate(),
-                  assetProps["BuildDate"].get<std::string>());
+                  sdbusplus::message::variant_ns::get<std::string>(
+                      assetProps["BuildDate"]));
         ASSERT_EQ(callout->manufacturer(),
-                  assetProps["Manufacturer"].get<std::string>());
-        ASSERT_EQ(callout->model(), assetProps["Model"].get<std::string>());
+                  sdbusplus::message::variant_ns::get<std::string>(
+                      assetProps["Manufacturer"]));
+        ASSERT_EQ(callout->model(),
+                  sdbusplus::message::variant_ns::get<std::string>(
+                      assetProps["Model"]));
         ASSERT_EQ(callout->partNumber(),
-                  assetProps["PartNumber"].get<std::string>());
+                  sdbusplus::message::variant_ns::get<std::string>(
+                      assetProps["PartNumber"]));
         ASSERT_EQ(callout->serialNumber(),
-                  assetProps["SerialNumber"].get<std::string>());
+                  sdbusplus::message::variant_ns::get<std::string>(
+                      assetProps["SerialNumber"]));
     }
 
     // Test a serialization failure due to a bad timestamp