power-supply: Fix variant build issue

The data type sdbusplus::message::variant has been removed.  Most of the
instances of usage within the phosphor-power repository were already
changed to std::variant.  However, there was one remaining instance that
needs to be fixed.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: Ie763530fa01a2917c220ff34537e5c8c3db8d25e
diff --git a/power-supply/power_supply.cpp b/power-supply/power_supply.cpp
index 7247c4c..9543afb 100644
--- a/power-supply/power_supply.cpp
+++ b/power-supply/power_supply.cpp
@@ -611,7 +611,7 @@
     using namespace sdbusplus::message;
 
     // Build the object map and send it to the inventory
-    using Properties = std::map<std::string, variant<std::string>>;
+    using Properties = std::map<std::string, std::variant<std::string>>;
     using Interfaces = std::map<std::string, Properties>;
     using Object = std::map<object_path, Interfaces>;
     Properties assetProps;