As noticed in #1066382, xserver-xorg-video-nouveau FTBFS with
-Werror=implicit-function-declaration, because wfbScreenInit is not
explicitly declared.  The file /usr/include/xorg/fb.h has a prototype
for it, but currently it is hidden behind #ifdef FB_ACCESS_WRAPPER.

Unfortunately, simply #defining FB_ACCESS_WRAPPER in
xserver-xorg-video-nouveau does not work because while it makes the
package build, Xorg then fails to start with a symbol lookup error.

The fix is to declare wfbScreenInit unconditionally, and this is already
in the Xserver master branch, but not in the server-21.1-branch.  The
upstream merge request mentions problems with the autoconf build system
which could be fixed by switching to meson.