Backends: SDL2: Fixed build for Emscripten and SDL_HAS_CAPTURE_AND_GLOBAL_MOUSE=0 platforms.

Amend e06b5df.

backends/imgui_impl_sdl2.cpp

@@ -667,6 +667,7 @@ static void ImGui_ImplSDL2_UpdateMouseData()
     SDL_Window* focused_window = SDL_GetKeyboardFocus();
     const bool is_app_focused = (bd->Window == focused_window);
 #else
+    SDL_Window* focused_window = bd->Window;
     const bool is_app_focused = (SDL_GetWindowFlags(bd->Window) & SDL_WINDOW_INPUT_FOCUS) != 0; // SDL 2.0.3 and non-windowed systems: single-viewport only
 #endif
     if (is_app_focused)