service/nvflinger: Mark FindVsyncEvent() as a const member function

This member function doesn't actually modify instance state, so it can
be marked as a const member function.

src/core/hle/service/nvflinger/nvflinger.cpp

@@ -78,7 +78,7 @@ u32 NVFlinger::FindBufferQueueId(u64 display_id, u64 layer_id) const {
     return layer.buffer_queue->GetId();
 }
 
-Kernel::SharedPtr<Kernel::ReadableEvent> NVFlinger::FindVsyncEvent(u64 display_id) {
+Kernel::SharedPtr<Kernel::ReadableEvent> NVFlinger::FindVsyncEvent(u64 display_id) const {
     return FindDisplay(display_id).vsync_event.readable;
 }
 

src/core/hle/service/nvflinger/nvflinger.h

@@ -67,7 +67,7 @@ public:
     u32 FindBufferQueueId(u64 display_id, u64 layer_id) const;
 
     /// Gets the vsync event for the specified display.
-    Kernel::SharedPtr<Kernel::ReadableEvent> FindVsyncEvent(u64 display_id);
+    Kernel::SharedPtr<Kernel::ReadableEvent> FindVsyncEvent(u64 display_id) const;
 
     /// Obtains a buffer queue identified by the ID.
     std::shared_ptr<BufferQueue> FindBufferQueue(u32 id) const;