Add macro to avoid the header file being included twice.

git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1633 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/xfr/fd_share.h

@@ -1,3 +1,6 @@
+#ifndef FD_SHARE_H_
+#define FD_SHARE_H_
+
 #include <stdlib.h>
 
 namespace isc {
@@ -17,3 +20,5 @@ send_fd(int sock, int fd);
 
 } // End for namespace xfr
 } // End for namespace isc
+
+#endif