anv: fix build with -Dvulkan-beta=true
Fixes:
In file included from ../src/intel/vulkan/genX_video.c:26:
../src/intel/genxml/gen_macros.h:85:19: error: no previous prototype for ‘gfx9_CmdEncodeVideoKHR’ [-Werror=missing-prototypes]
85 | # define genX(x) gfx9_##x
The empty encode stubs were not forward declared.