Fix dtor declaration for clang-6

Reviewed By: smeenai

Differential Revision: D7690428

fbshipit-source-id: 4b02a6f6cbccda3446f0a19b1b35c266a33560bd

ReactCommon/fabric/uimanager/Scheduler.h

@@ -29,7 +29,7 @@ class Scheduler:
 
 public:
   Scheduler();
-  ~Scheduler();
+  virtual ~Scheduler();
 
 #pragma mark - Root Nodes Managerment
 

ReactCommon/fabric/uimanager/SchedulerDelegate.h

@@ -16,6 +16,9 @@ namespace react {
  */
 class SchedulerDelegate {
 public:
+
+  virtual ~SchedulerDelegate() = default;
+
   /*
    * Called right after Scheduler computed (and laid out) a new updated version
    * of the tree and calculated a set of mutation instructions which are