Fix building with gcc-4.7

https://bugs.gentoo.org/show_bug.cgi?id=420959
http://www.openldap.org/its/index.cgi/Incoming?id=7304;page=16  #ITS 7304

Patch written by Kacper Kowalik <xarthisius@gentoo.org>
--- a/contrib/ldapc++/src/SaslInteractionHandler.cpp
+++ b/contrib/ldapc++/src/SaslInteractionHandler.cpp
@@ -16,6 +16,7 @@
 
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 #include "SaslInteractionHandler.h"
 #include "SaslInteraction.h"
 #include "debug.h"