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: I1f4272333853a3da9c27cd927ee0997c81bc6330
diff --git a/include/watchdog_event_monitor.hpp b/include/watchdog_event_monitor.hpp
index 57ba24e..4c53e06 100644
--- a/include/watchdog_event_monitor.hpp
+++ b/include/watchdog_event_monitor.hpp
@@ -241,8 +241,7 @@
     std::shared_ptr<sdbusplus::asio::connection> conn)
 {
     auto watchdogEventMatcherCallback = [conn](sdbusplus::message_t& msg) {
-        std::string expiredAction;
-        msg.read(expiredAction);
+        auto expiredAction = msg.unpack<std::string>();
 
         std::string_view action = expiredAction;
         action.remove_prefix(