PEL: Remove use of deprecated mem_fun

Replace it with mem_fn.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I1b223f2de54f9b6c6883fb539276f9750575ab96
diff --git a/extensions/openpower-pels/host_notifier.cpp b/extensions/openpower-pels/host_notifier.cpp
index a89af1d..427ec79 100644
--- a/extensions/openpower-pels/host_notifier.cpp
+++ b/extensions/openpower-pels/host_notifier.cpp
@@ -55,9 +55,8 @@
 
     // Subscribe to be told about host state changes.
     _dataIface.subscribeToHostStateChange(
-        subscriptionName,
-        std::bind(std::mem_fun(&HostNotifier::hostStateChange), this,
-                  std::placeholders::_1));
+        subscriptionName, std::bind(std::mem_fn(&HostNotifier::hostStateChange),
+                                    this, std::placeholders::_1));
 
     // Set the function to call when the async reponse is received.
     _hostIface->setResponseFunction(