gl: fix fallback definition of EGL_DEFAULT_DISPLAY

gl: fix fallback definition of EGL_DEFAULT_DISPLAY

Compiling fbdev backend on RaspberryPi caused the following warning:

compositor-fbdev.c: In function 'fbdev_compositor_create':
compositor-fbdev.c:929:6: warning: passing argument 2 of
'gl_renderer->create' makes integer from pointer without a cast [enabled
by default]
compositor-fbdev.c:929:6: note: expected 'EGLNativeDisplayType' but
argument is of type 'void *'

Fix the definition of EGL_DEFAULT_DISPLAY to match the definition in
EGL/egl.h (of Mesa).

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>