Make default graphics backend not show up as empty

Fixes https://bugs.dolphin-emu.org/issues/12245.

I considered making a change to DolphinQt instead of
the core, but then additional effort would've been
required to add the same fix to the Android GUI once
we start using the new config system there.

Source/Core/VideoCommon/VideoBackendBase.h

@@ -59,6 +59,7 @@ public:
   u32 Video_GetQueryResult(PerfQueryType type);
   u16 Video_GetBoundingBox(int index);
 
+  static std::string GetDefaultBackendName();
   static void PopulateList();
   static void ClearList();
   static void ActivateBackend(const std::string& name);