Fix -Wundef warnings due to use of unguarded SDL_VIDEO_DRIVER_DUMMY

src/video/SDL_video.c

@@ -124,7 +124,7 @@ static VideoBootStrap *bootstrap[] = {
 #ifdef SDL_VIDEO_DRIVER_NGAGE
     &NGAGE_bootstrap,
 #endif
-#if SDL_VIDEO_DRIVER_DUMMY
+#ifdef SDL_VIDEO_DRIVER_DUMMY
     &DUMMY_bootstrap,
 #if SDL_INPUT_LINUXEV
     &DUMMY_evdev_bootstrap,

src/video/dummy/SDL_nullevents.c

@@ -20,7 +20,7 @@
 */
 #include "SDL_internal.h"
 
-#if SDL_VIDEO_DRIVER_DUMMY
+#ifdef SDL_VIDEO_DRIVER_DUMMY
 
 /* Being a null driver, there's no event stream. We just define stubs for
    most of the API. */

src/video/dummy/SDL_nullframebuffer.c

@@ -20,7 +20,7 @@
 */
 #include "SDL_internal.h"
 
-#if SDL_VIDEO_DRIVER_DUMMY
+#ifdef SDL_VIDEO_DRIVER_DUMMY
 
 #include "../SDL_sysvideo.h"
 #include "SDL_nullframebuffer_c.h"

src/video/dummy/SDL_nullvideo.c

@@ -20,7 +20,7 @@
 */
 #include "SDL_internal.h"
 
-#if SDL_VIDEO_DRIVER_DUMMY
+#ifdef SDL_VIDEO_DRIVER_DUMMY
 
 /* Dummy SDL video driver implementation; this is just enough to make an
  *  SDL-based application THINK it's got a working video driver, for