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/utils.cpp b/utils.cpp
index 4f59f6c..bfe924c 100644
--- a/utils.cpp
+++ b/utils.cpp
@@ -543,7 +543,7 @@
     if (rc <= 0)
     {
         log<level::ERR>("inet_pton failed:",
-                        entry("SUBNETMASK=%s", subnetMask));
+                        entry("SUBNETMASK=%s", subnetMask.c_str()));
         return 0;
     }
 
@@ -556,7 +556,7 @@
     else
     {
         log<level::ERR>("Invalid Mask",
-                        entry("SUBNETMASK=%s", subnetMask));
+                        entry("SUBNETMASK=%s", subnetMask.c_str()));
         return 0;
     }
 }