Include what you use: <errno.h>.

Change-Id: Ic970f6bf4558e8e0ef3d7e480c2532c102abfd7c
diff --git a/client/FwmarkClient.cpp b/client/FwmarkClient.cpp
index db2009f..1e98c40 100644
--- a/client/FwmarkClient.cpp
+++ b/client/FwmarkClient.cpp
@@ -16,6 +16,7 @@
 
 #include "FwmarkClient.h"
 
+#include <errno.h>
 #include <stdlib.h>
 #include <sys/socket.h>
 #include <sys/un.h>