use sdbusplus unpack syntax

Rather than defining a variable and then reading it from a message,
sdbusplus also supports directly unpack-ing from the message.  Use
this syntax instead as it is more efficient and succinct.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I23902026dee367a8cbf0c2725725c4ed3ca4d2bb
diff --git a/app/watchdog_service.cpp b/app/watchdog_service.cpp
index da06041..29f81c9 100644
--- a/app/watchdog_service.cpp
+++ b/app/watchdog_service.cpp
@@ -117,8 +117,8 @@
     try
     {
         auto response = bus.call(request);
-        std::variant<T> value;
-        response.read(value);
+        auto value = response.unpack<std::variant<T>>();
+
         return std::get<T>(value);
     }
     catch (const std::exception& e)