Add missing <string.h> include.

Change-Id: I14ea45e98b8271d6b53ac86e92ad3b5c7dac8f75
diff --git a/client/FwmarkClient.cpp b/client/FwmarkClient.cpp
index 1e98c40..4e02d58 100644
--- a/client/FwmarkClient.cpp
+++ b/client/FwmarkClient.cpp
@@ -18,6 +18,7 @@
 
 #include <errno.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <unistd.h>