[BUILD] make ebtree headers multiple-include compatible

ebtree did not support being included multiple times.
diff --git a/include/common/ebtree.h b/include/common/ebtree.h
index 7a595b9..ebd1e91 100644
--- a/include/common/ebtree.h
+++ b/include/common/ebtree.h
@@ -246,6 +246,8 @@
 
  */
 
+#ifndef _COMMON_EBTREE_H
+#define _COMMON_EBTREE_H
 
 #include <stdlib.h>
 
@@ -716,6 +718,7 @@
 void eb_delete(struct eb_node *node);
 REGPRM1 struct eb_node *eb_insert_dup(struct eb_node *sub, struct eb_node *new);
 
+#endif /* _COMMON_EBTREE_H */
 
 /*
  * Local variables: