Fixes warning: implicit declaration of function

nv_driver.c:1443:9: warning: implicit declaration of function ‘wfbScreenInit’; did you mean ‘fbScreenInit’? [-Wimplicit-function-declaration] ret = wfbScreenInit(pScreen, FBStart, pScrn->virtualX, ^~~~~~~~~~~~~ fbScreenInit

In commit https://gitlab.freedesktop.org/sergiomb/xf86-video-nouveau/-/commit/794a27724fb9ab173dc7ea8062730797e1b4de04
"nv50: use libwfb for pixmap access when driver pixmaps enabled", authored 15 years ago, it is missing the declaration of wfbScreenInit

This warning was harmless until GCC and Modern C make implicit "declaration of function" an error.

We got one commit that attempts to fix this [1] which is in Xwayland branch but not in xorg-x11-server branch and IMO should be reverted.

[1]
https://gitlab.freedesktop.org/xorg/xserver/-/commit/34a430a16ec2cb9851b1f2bc7fc2117e19b9fcc7

Signed-off-by: Sérgio M. Basto <sergio@serjux.com>