xserver-xorg-video-nouveau 1:1.0.17-3 FTBFS on armhf

[Impact]

xserver-xorg-video-nouveau 1:1.0.17-3 fails to build on armhf:

```
libtool: compile: gcc -DHAVE_CONFIG_H -I. -I../../src -I.. -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64 -Wdate-time -D_FORTIFY_SOURCE=3 -I/usr/include/xorg -fvisibility=hidden -I/usr/include/pixman-1 -I/usr/include/X11/dri -I/usr/include/libdrm -I/usr/include/libdrm -I/usr/include/libdrm/nouveau -I/usr/include/libdrm -g -O2 -Werror=implicit-function-declaration -ffile-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -fno-stack-clash-protection -fdebug-prefix-map=/<<PKGBUILDDIR>>=/usr/src/xserver-xorg-video-nouveau-1:1.0.17-3 -Wall -I/usr/include/xorg -fvisibility=hidden -I/usr/include/pixman-1 -I/usr/include/X11/dri -I/usr/include/libdrm -c ../../src/nv_shadow.c -fPIC -DPIC -o .libs/nv_shadow.o
../../src/nv_driver.c: In function ‘NVScreenInit’:
../../src/nv_driver.c:1451:23: error: implicit declaration of function ‘wfbScreenInit’; did you mean ‘fbScreenInit’? [-Werror=implicit-function-declaration]
 1451 | ret = wfbScreenInit(pScreen, FBStart, pScrn->virtualX,
      | ^~~~~~~~~~~~~
      | fbScreenInit
```

This failure is fixed upstream in xorg-server via https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1114

[Test Plan]

The package needs to build for armhf

[Where problems could occur]

The patch adds function prototypes explicitly. If the function signature does not match, the code will still compile, but the behaviour will be undefined.
This would not happen as the package only needs the inline definitions for armhf.
On other architectures the header is included correctly and the compiler will produce a 'conflicting types' error.

[Other Info]

n/a