[BUILD] make ebtree headers multiple-include compatible

ebtree did not support being included multiple times.
diff --git a/include/common/ebpttree.h b/include/common/ebpttree.h
index 4908f81..be164ad 100644
--- a/include/common/ebpttree.h
+++ b/include/common/ebpttree.h
@@ -17,6 +17,9 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
+#ifndef _COMMON_EBPTTREE_H
+#define _COMMON_EBPTTREE_H
+
 #include "ebtree.h"
 
 
@@ -315,3 +318,4 @@
 	return new;
 }
 
+#endif /* _COMMON_EBPTTREE_H */