Fix loading the driver when compiled for xorg 1.20.7

[grembo]

When loading scfb on xorg 1.20.7 it fails with an
undefined symbol error:

scfb trace: probe start
scfb trace: probe done
scfb: PreInit 0
scfb: PreInit done
scfb: ScfbScreenInit 0
bitsPerPixel=32, depth=24, defaultVisual=TrueColor
mask: ff0000,ff00,ff, offset: 16,8,0
mmap returns: addr 0x805a10000 len 0x640000, fd 12, off 0
scfb: ScfbSave 0
scfb: ScfbSave done
scfb: ScfbScreenInit done
ld-elf.so.1: /usr/local/lib/xorg/modules/drivers/scfb_drv.so: Undefined
symbol "shadowUpdatePackedWeak"

Disabling ShadowFB makes the driver load, but breaks output.

With this fix, the driver loads and was reported to work
properly (at least for "normal" daily use).

The fix was inspired by a similar change in fbdev:
https://github.com/freedesktop/xf86-video-fbdev/commit/2673e727063fe28310836f1e6e9eda552930218a

[evadot]

That's also what wsfb does : https://gitlab.freedesktop.org/xorg/driver/xf86-video-wsfb/blob/master/src/wsfb_driver.c#L632

And sorry about this as I have this in my local tree ...

[rayddteam]

Thanks, guys!