fix: put SDL_WndProc behind _WIN32 ifdef

[briaguya-ai]

This was added in #365

It is only used in _WIN32 #ifdef blocks

libultraship/src/graphic/Fast3D/gfx_sdl2.cpp

Lines 290 to 303 in ef0c348

	#ifdef _WIN32
	static LRESULT CALLBACK gfx_sdl_wnd_proc(HWND h_wnd, UINT message, WPARAM w_param, LPARAM l_param) {
	switch (message) {
	case WM_GETDPISCALEDSIZE:
	// Something is wrong with SDLs original implementation of WM_GETDPISCALEDSIZE, so pass it to the default
	// system window procedure instead.
	return DefWindowProc(h_wnd, message, w_param, l_param);
	default:
	// Pass anything else to SDLs original window procedure.
	return CallWindowProc((WNDPROC)SDL_WndProc, h_wnd, message, w_param, l_param);
	}
	return 0;
	};
	#endif

libultraship/src/graphic/Fast3D/gfx_sdl2.cpp

Lines 366 to 374 in ef0c348

	#ifdef _WIN32
	// Get Windows window handle and use it to subclass the window procedure.
	// Needed to circumvent SDLs DPI scaling problems under windows (original does only scale *sometimes*).
	SDL_SysWMinfo wmInfo;
	SDL_VERSION(&wmInfo.version);
	SDL_GetWindowWMInfo(wnd, &wmInfo);
	HWND hwnd = wmInfo.info.win.window;
	SDL_WndProc = SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR)gfx_sdl_wnd_proc);
	#endif