[core] make RunLoop::schedule public

include/mbgl/util/run_loop.hpp

@@ -62,6 +62,12 @@ public:
         push(task);
         return std::make_unique<WorkRequest>(task);
     }
+                    
+    void schedule(std::weak_ptr<Mailbox> mailbox) override {
+        invoke([mailbox] () {
+            Mailbox::maybeReceive(mailbox);
+        });
+    }
 
     class Impl;
 
@@ -72,12 +78,6 @@ private:
 
     void push(std::shared_ptr<WorkTask>);
 
-    void schedule(std::weak_ptr<Mailbox> mailbox) override {
-        invoke([mailbox] () {
-            Mailbox::maybeReceive(mailbox);
-        });
-    }
-
     void withMutex(std::function<void()>&& fn) {
         std::lock_guard<std::mutex> lock(mutex);
         fn();