Fix std::variant usage

We need to use the std::get() interface instead of the mapbox .get()
member interface for variant compatibility.

Change-Id: I6c7843ea0cb765332457273867b5df6b78ca72a2
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/sdbusplus.hpp b/sdbusplus.hpp
index 41c7fe2..8a5b2f4 100644
--- a/sdbusplus.hpp
+++ b/sdbusplus.hpp
@@ -339,7 +339,7 @@
             }
             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 with mapper lookup. */
@@ -433,7 +433,7 @@
             }
             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. */