Support custom callback function on timer expiration

Change-Id: I39b32d608ef342d63c57cbc1902e927fb39861c7
Signed-off-by: Andrew Geissler <andrewg@us.ibm.com>
diff --git a/softoff/timer.cpp b/softoff/timer.cpp
index c626536..4e8fd9a 100644
--- a/softoff/timer.cpp
+++ b/softoff/timer.cpp
@@ -51,6 +51,12 @@
     auto timer = static_cast<Timer*>(userData);
     timer->expired = true;
 
+    // Call optional user call back function if available
+    if(timer->userCallBack)
+    {
+        timer->userCallBack();
+    }
+
     log<level::INFO>("Timer expired");
     return 0;
 }