src/video/kmsdrm/SDL_kmsdrmvideo.c: fix build

Build is broken without EGL since version 2.0.12 and
https://hg.libsdl.org/SDL/rev/72cc4740dec2:

/home/giuliobenetti/autobuild/run/instance-1/output-1/build/sdl2-2.0.12/src/video/kmsdrm/SDL_kmsdrmvideo.c: In function 'KMSDRM_CreateSurfaces':
/home/giuliobenetti/autobuild/run/instance-1/output-1/build/sdl2-2.0.12/src/video/kmsdrm/SDL_kmsdrmvideo.c:394:5: error: unknown type name 'EGLContext'
     EGLContext egl_context;
     ^

Fixes:
 - http://autobuild.buildroot.org/results/fafd20a01591032662f9ca025fcea3478239cf3c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

src/video/kmsdrm/SDL_kmsdrmvideo.c

@@ -391,7 +391,9 @@ KMSDRM_CreateSurfaces(_THIS, SDL_Window * window)
     Uint32 height = dispdata->mode.vdisplay;
     Uint32 surface_fmt = GBM_FORMAT_XRGB8888;
     Uint32 surface_flags = GBM_BO_USE_SCANOUT | GBM_BO_USE_RENDERING;
+#if SDL_VIDEO_OPENGL_EGL
     EGLContext egl_context;
+#endif
 
     if (!KMSDRM_gbm_device_is_format_supported(viddata->gbm, surface_fmt, surface_flags)) {
         SDL_LogWarn(SDL_LOG_CATEGORY_VIDEO, "GBM surface format not supported. Trying anyway.");