[Qt] Header cleanup (#6477)

Make some headers private, so we only commit with the QML API.

platform/qt/src/run_loop.cpp

@@ -4,11 +4,10 @@
 
 #include <QCoreApplication>
 
+#include <cassert>
 #include <functional>
 #include <utility>
 
-#include <cassert>
-
 namespace {
 
 using namespace mbgl::util;