fb: Declare wfbFinishScreenInit, wfbScreenInit for !FB_ACCESS_WRAPPER
xf86-video-nouveau calls wfbScreenInit without defining FB_ACCESS_WRAPPER (which has other unintended side effects). Presently, this compiles and links because compilers still support implicit function declarations, but this is going to change fairly soon. This seems to be the most straightforward change to keep the driver building.
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