[BUILD] make ebtree headers multiple-include compatible

ebtree did not support being included multiple times.
diff --git a/include/common/eb64tree.h b/include/common/eb64tree.h
index 242e2b1..9a069ca 100644
--- a/include/common/eb64tree.h
+++ b/include/common/eb64tree.h
@@ -17,6 +17,9 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
+#ifndef _COMMON_EB64TREE_H
+#define _COMMON_EB64TREE_H
+
 #include "ebtree.h"
 
 
@@ -532,3 +535,4 @@
 	return new;
 }
 
+#endif /* _COMMON_EB64TREE_H */