Use s.c_str() in log messages

Fixes issues with data passed to phosphor::logging::log(entry()).
This is part of a series of commits that resolves openbmc 2905.

Tested: static_assert only

Change-Id: I9fac771f54e4acbec97ce1360c106b6e3eb4fe9d
Signed-off-by: Joseph Reynolds <jrey@us.ibm.com>
diff --git a/softoff/softoff.cpp b/softoff/softoff.cpp
index f76cd4f..d76750a 100644
--- a/softoff/softoff.cpp
+++ b/softoff/softoff.cpp
@@ -86,9 +86,9 @@
         else
         {
             log<level::INFO>("Timer started waiting for host to shutdown",
-                    entry("TIMEOUT_IN_MSEC=%llu",
-                        duration_cast<milliseconds>(seconds
-                            (IPMI_HOST_SHUTDOWN_COMPLETE_TIMEOUT_SECS))));
+                entry("TIMEOUT_IN_MSEC=%llu",
+                      (duration_cast<milliseconds>(seconds
+                        (IPMI_HOST_SHUTDOWN_COMPLETE_TIMEOUT_SECS))).count()));
         }
     }
     else