Backends: SDL3: add default case to fix warnings. (#7763)

backends/imgui_impl_sdl2.cpp

@@ -287,6 +287,7 @@ static ImGuiKey ImGui_ImplSDL2_KeyEventToImGuiKey(SDL_Keycode keycode, SDL_Scanc
         case SDLK_F24: return ImGuiKey_F24;
         case SDLK_AC_BACK: return ImGuiKey_AppBack;
         case SDLK_AC_FORWARD: return ImGuiKey_AppForward;
+        default: break;
     }
     return ImGuiKey_None;
 }