Rename RB timers to timer_tree

include/ucore/ucore_timers.h

@@ -61,7 +61,7 @@ struct UCTimer {
 
 /** Timer engine. Must be initialized onnce.*/
 struct UCTimers {
-    RBTree timers;
+    RBTree timer_tree;
     size_t seq; //used to generate unique timers
     TAILQ_HEAD(, UCTimer) ready_list; //pending timers to be fired while inside uc_timers_run
 };