Profiler: Fix unmovable splitter

Set preferred height instead of fixed height to allow the splitter to
move.

The splitter respects set_fixed_height() after a808cfa "LibGUI+
Applications: Govern Splitter resizing by opportunistic growth", and
that caused the splitter in Profiler to stop working.

Userland/DevTools/Profiler/TimelineContainer.cpp

@@ -24,7 +24,7 @@ TimelineContainer::TimelineContainer(GUI::Widget& header_container, TimelineView
  update_widget_positions();
 
  int initial_height = min(300, timeline_view.height() + 16 + frame_thickness() * 2);
- set_fixed_height(initial_height);
+ set_preferred_height(initial_height);
 
  m_timeline_view->on_scale_change = [this] {
  update_widget_sizes();