Attn: Additional tracing for attention handler

Added some tracing that has proved to be beneficial for debugging recent
issues with the pdbg library.

Signed-off-by: Ben Tyner <ben.tyner@ibm.com>
Change-Id: Id4c034a159170439c498129c013d45fbe08cdaa0
diff --git a/attn/attn_monitor.cpp b/attn/attn_monitor.cpp
index c114bdf..053d615 100644
--- a/attn/attn_monitor.cpp
+++ b/attn/attn_monitor.cpp
@@ -24,6 +24,7 @@
             }
             else
             {
+                trace<level::INFO>("Attention GPIO active");
                 handleGPIOEvent(); // gpio trigger detected
             }
             return;